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4 [2 + 2]-Photocycloadditon of α,β-Unsaturated Carboxylic Acid Derivatives (Substrate Classes A4, A5, and A6)

4 [2 + 2]-Photocycloadditon of α,β-Unsaturated Carboxylic Acid Derivatives (Substrate Classes A4, A5, and A6)

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j 6 Formation of a Four Membered Ring



190



1. Cl



Cl



hν (Hg lamp)

−15 °C (MeCN)

2. Zn, μw (EtOH)



O

O



64%



PivO



O H



H

O



O

PivO



79



80

1. Cl



O



81



Cl



hν (Hg lamp)

−20 °C (MeCN)

2. Zn, Ac2O (PhMe)



O



O



O



O



O



O



O



75%

PivO



O



PivO



O



82



83



OH



84



Scheme 6.30



sequence, the photocycloaddition proved under similar conditions to be equally

diastereoselective (d.r. ¼ 91/9), yielding after reduction the major diastereoisomer

83. The two quaternary stereogenic centers established in the photochemical

reactions were retained in the further synthetic route, which eventually led to the

sesquiterpene ( ) merrilactone A (84) [88].

The crossed intramolecular [2 ỵ 2] photocycloaddition of allenes to a,b unsat

urated g lactones has been extensively studied by Hiemstra et al. in an approach to

racemic solanoeclepin A (87). The sensitized irradiation of butenolide 85 in a 9 : 1

mixture of benzene and acetone, for example, led selectively to the strained photo

cycloadduct 86 (Scheme 6.31) [89]. The facial diastereoselectivity is determined

by the stereogenic center, to which the allene is attached. The carbon atom in a

position to the carbonyl carbon atom is attacked from its re face, forming a bond to

the tertiary allene carbon atom, while the b carbon atom is being connected to the

internal allene carbon atom by a si face attack. The method allows facial diaster

eocontrol over three contiguous stereogenic centers in the bicyclo[2.1.1]heptane

part of the natural product.

COOH

O

O



O

O



hν (λ = 300nm)

r.t (PhH/ac)



OBn

OH

O



60%



O



O



O



HO



O



O



MeO



BnO

85

Scheme 6.31



86



87



O



6.4 [2 ỵ 2] Photocycloadditon of a,b Unsaturated Carboxylic Acid Derivatives



OH

O



O

O



O



hν (λ > 290 nm)

r.t (ac)



O

O



HO



O

AcO

O



O



50%

d.r. = 83/17



H



H



O



H



HO



OH

O



OH

88



OH



89



90



Scheme 6.32



Another natural product, which contains a cyclobutane ring, is the hexacyclic

diterpene bielschowskysin (90), the absolute configuration of which has not yet been

determined. The retrosynthetic disconnection into a [2 ỵ 2] photocycloaddition

precursor is easily recognized in its structure and appears to be possible in a forward

direction (Scheme 6.32) [90]. At least the conversion of butenolide 88 into tetracyclic

product 89 was possible. The configuration of the exocyclic double bond was not

retained in the [2 ỵ 2] photocycloaddition because an isomerization was observed

during irradiation. In addition, since the reaction is likely to proceed via 1,4 diradical

intermediates, it is not expected to be stereospecific. Remarkably, diastereoisomer 89

was the predominant isomer with the other diastereomer, which was formed in

minor amounts, being an epimer of 89 with opposite configuration at the spiro

center. In close analogy to previously mentioned examples (Schemes 6.7, 6.20

and 6.31) the facial diastereoselectivity is controlled by the endocyclic stereogenic

center adjacent to the double bond.

The photocycloaddition chemistry of a,b unsaturated d lactones is similar to

the chemistry of g lactones. Complications arise as with cyclohexenones because

anti addition to the a,b unsaturated double bond can occur, particularly in the

intermolecular addition mode. Even if one product prevails, intermolecular

[2 ỵ 2] photocycloaddition reactions are often sluggish. Despite the fact that alkene

92, for example, was employed in a twofold excess relative to dihydropyranone 91,

the reaction delivered only 32% of the desired product 93 (43% based on recovered

starting material; Scheme 6.33). The relative product configuration, which was

established by X ray crystallography, came as a surprise because the lactone apparently



O

N

O

O



hν (Hg lamp)

r.t. (EtOAc)

43%



91

Scheme 6.33



92

O

O



HOOC

H H



O



COOH



O

N



NH

O



H

93



94



j191



j 6 Formation of a Four Membered Ring



192



adds onto the sterically hindered concave face of the proline derived dihydropyr

role 92. The relative configuration within the doubly anellated cyclobutane was

expectedly cis anti cis. The experiments were conducted in connection with a study

towards a total synthesis of ( ) kainic acid (94) [91].

O



hν (λ = 254 nm)

r.t. (CH2Cl2)



HN

m



O

HN

m



m = 1 61%

m = 2 52%



95



96



Scheme 6.34



6.4.1.2 a,b-Unsaturated Lactams

Lactams of type A4 have been successfully employed in intramolecular [2 ỵ 2]

photocycloaddition reactions. Direct excitation has been found most useful to achieve

sufficient conversion in these reactions. The reaction of b substituted a,b unsatu

rated lactams 95 in CH2Cl2 gave the expected products 96 of five membered ring

closure in decent yields (Scheme 6.34) [92].

A stereogenic center in g position provides good diastereofacial control in

g lactams, with the regioselectivity of the photocycloaddition being dependent on

the chain length of the tether. The N tert butoxycarbonyl(Boc) protected lactams

97 were subjected to an intramolecular [2 ỵ 2] photocycloaddition in acetonitrile as

the solvent (Scheme 6.35). The substrate with the allyl side chain (n ¼ 1) formed

predominantly the crossed photocycloaddition product 98 (52%), but minor

quantities of the straight photocycloaddition product 99 were also observed. In line

with the rule of five, the situation was reversed in the butenyl substituted sub

strate (n ¼ 2), with 99 prevailing over 98. The pentenyl substituted substrate 97

(n ¼ 3) yielded a single product 99, presumably via an initial six membered ring

formation, followed by subsequent ring closure of the resulting 1,4 biradical to the

cyclobutane [93].

Pyridones can react photochemically along several reaction channels [94]. Besides

[4 ỵ 4] photodimerization and [4p] ring closure, [2 ỵ 2] photocycloaddition reac

tions are possible in an a,b or in a g,d mode relative to the carbonyl carbon atom.

With regard to the former reaction pathway, the [2 ỵ 2] photocycloaddition of olens

to 4 alkoxypyridones appears to be synthetically most useful (vide infra).

O

Boc N



hν (λ = 254 nm)

r.t. (MeCN)



O



O



Boc N



+



n



Boc N



n



97

Scheme 6.35



98



n



99



n=1

n=2

n=3



98

52%

8%





99

10%

62%

61%



6.4 [2 þ 2] Photocycloadditon of a,b Unsaturated Carboxylic Acid Derivatives



O

O



MeO



O

COOEt



OMe



hν (Hg lamp)

r.t. (PhH)

94%

d.r. = 91/9



Ph HO



COOEt



O



O

O



MeO



OMe



100



HO



OMe

102



101



Scheme 6.36



6.4.1.3 Coumarins

The direct irradiation of the parent coumarin in the presence of alkenes results only

in an inefcient photodimerization and [2 ỵ 2] photocycloaddition. Lewis acid co

ordination appears to increase the singlet state lifetime, and leads to improved yields

in the stereospecic [2 ỵ 2] photocycloaddition [95]. Alternatively, triplet sensitiza

tion can be employed to facilitate a [2 þ 2] photocycloaddition. Yields of intra

molecular [2 þ 2] photocycloadditions remain, however, even with electron rich

alkenes in the medium range at best. The preference for HT addition and for

formation of the exo product is in line with mechanistic considerations discussed

earlier for other triplet [2 ỵ 2] photocycloadditions [96, 97]. Substituted coumarins

were found to react more efficiently than the parent compound, even under

conditions of direct irradiation. 3 Substituted coumarins, for example, 3 methoxy

carbonylcoumarin [98], are most useful and have been exploited extensively. The

reaction of 3 ethoxycarbonylcoumarin (100) with 3 methyl 1 butene yielded cleanly

the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo product

(d.r. ¼ 91/9). Product 101 underwent a ring opening/ring closure sequence upon

treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur

an, which was further converted into the natural product (Ỉ) linderol A (102) [99].

Coumarin photochemistry has been recently employed to demonstrate that a

frozen axial chirality can be used to induce the absolute configuration of stereogenic

centers. Coumarin 103 was obtained as a single atropisomer by spontaneous

crystallization (Scheme 6.37). Upon warming powdered crystals of 103 in MeOH

to 20  C, sensitized [2 ỵ 2] photocycloaddition to ethyl vinyl ether gave the almost

enantiomerically pure products 104. The approach to the coumarin double bond

occurred preferentially from the less shielded face to which the amide carbonyl group



O



O

NEt2



O



restricted

rotation



103

Scheme 6.37



O

OEt, Ph2CO

hν (λ = 365 nm)

−20 °C (MeOH)

98%

endo/exo = 75/25



j193



O



CONEt2



O



+

H



OEt



104a (98% ee)



CONEt2



O

H



OEt



104b (97% ee)



j 6 Formation of a Four Membered Ring



194



O

O



O

O



NH



N



O



hν (Hg lamp)

r.t. (MeCN/MeOH)

64%



MeO



O



N



HN



O

O



MeO

105



106



Scheme 6.38



points. Desired homochiral crystals for this process could be selectively prepared by

the addition of a corresponding seed crystal during the crystallization process [100].

Intramolecular [2 ỵ 2] photocycloadditions of coumarins proceed with regard

to regio and stereoselectivity along the guidelines previously discussed for other

a,b unsaturated carbonyl compounds. In a recent example, the products of an

Ugi four component reaction, such as amide 105, were converted photochemically

into uniquely shaped 3 azabicyclo[4.2.0]octan 4 ones, such as cyclobutane 106

(Scheme 6.38) [101].

6.4.1.4 Quinolones

Due to a very efficient singlet to triplet state intersystem crossing, the [2 ỵ 2]

photocycloaddition chemistry of 2 quinolones can be initiated easily by direct

excitation at 300 350 nm [102]. The addition of a sensitizer is not required. The

parent compound has been first employed in a [2 þ 2] photocycloaddition as early as

1968 [103]. With regard to regio and stereoselectivity, 2 quinolone (107) behaves as

expected, exhibiting a preference for HTproduct formation with electron rich olefins,

such as 1,1 dimethoxyethene (Scheme 6.39, DMA ¼ N,N dimethylacetamide). The

highly efficient reaction delivers product 108 quantitatively [104]. The preference for

OMe

O



O



MeO

hν (λ = 350 nm)

r.t. (DMA)



HN



HN

OMe

OMe



quant.

107



108



O

HN



O

OAc



hν (λ > 290 nm)

r.t. (MeOH)



OAc



HN



63%

109

Scheme 6.39



110



6.4 [2 þ 2] Photocycloadditon of a,b Unsaturated Carboxylic Acid Derivatives



OTMS

O



hν (λ = 350 nm)

r.t. (PhMe)



HN



O



MeOOC



HN

COOMe

OTMS



98%

NBnBoc

111



O



HN



N



NBnBoc

112



113



Scheme 6.40



exo product formation, as observed for example with a d.r. of 75/24 for the reaction

of 107 and cyclopentene, and as typically observed in enone photochemistry, is

reversed for 3 acyloxyquinolones such as 3 acetoxyquinolone (109). Reaction with

cyclopentene resulted in a product mixture, in which the endo product (cis syn cis)

prevailed (d.r. ¼ 70/30), and addition to cyclohexene resulted almost exclusively in

the cis syn cis product 110 [105]. Possibly, hydrogen bonds to the protic solvent

methanol enlarge the size of the acetoxy group disfavoring the formation of the

exo product.

4 Alkylquinolones react with acrylates regioselectively to deliver the formal HT

products [106]. According to the 1,4 biradical model (Scheme 6.2), radical stabiliza

tion in the benzylic position overrides other electronic effects. A recent application of

quinolone photocycloaddition chemistry to natural product synthesis is depicted in

Scheme 6.40. The silylenolether derived from methyl pyruvate underwent a clean

addition reaction to the N,N diprotected 4 (20 aminoethyl) substituted quinolone

111. The stereoselective and high yielding reaction delivered a single product with

the larger substituents (aryl/COOMe) trans positioned. Upon treatment with K2CO3

in MeOH, product 112 underwent a ring enlargement to a five membered ring (retro

benzilic acid rearrangement) under retention of configuration at the stereogenic

centers in a and b position. Subsequent key steps en route to the formation of the

pentacyclic monoterpenoid indole alkaloid (Ỉ) meloscine (113) were a reductive

amination, a Claisen rearrangement, and a ring closing metathesis [107].

Recent interest in the use of N unsubstituted 2 quinolones stems from the fact,

that they coordinate effectively to chiral lactam based templates via two hydrogen

bonds. The prototypical template to be used in photochemical reactions is compound

115, which can be readily prepared from Kemp’s triacid [108]. The template is

transparent at a wavelength l ! 290 nm, and can be nicely used in stoichiometric

amounts for enantioselective photochemical and radical reactions [109]. Conditions

which favor hydrogen bonding (nonpolar solvent, low temperature) are required to

achieve an efficient association of a given substrate. The intramolecular [2 ỵ 2]

photocycloaddition of 4 alkylquinolone 114 proceeded in the presence of 115 with

excellent enantioselectivity, and delivered product 116 as the exclusive stereoisomer

(Scheme 6.41) [110]. Application of the enantiomer ent 115 of complexing agent 115

to the reaction 111 ! 112 depicted in Scheme 6.40 enabled enantioselective access

to ( ỵ ) meloscine [111].



j195



j 6 Formation of a Four Membered Ring



196



HN

O



O O

N



HN



O



115



hν (λ > 290 nm)

−60 °C (PhMe)

N

Boc



HN



N



78%

93% ee



Boc



114



116



Scheme 6.41



6.4.1.5 Maleic Anhydride and Derivatives

Maleic anhydride (117) belongs to the rst compounds, the [2 ỵ 2] photocycloaddi

tion reactions of which were extensively explored [112]. It is preferably converted

to the corresponding cyclobutanes by irradiation in the presence of a sensitizer,

for example, benzophenone, allowing the addition of a plethora of alkenes

(Scheme 6.42). In a recent application the photocycloaddition product 118 of maleic

anhydride and 1,4 dichloro 2 butene was converted into the marine alkaloid

(Ỉ) sceptrin (119) [113].

Substituted maleic anhydrides have been directly excited, but sensitization may

also be used. In some cases the first method is better, and in some cases the second.

In an approach to merrilactone A, which is closely related to the earlier mentioned

synthesis (Scheme 6.30), 2,3 dimethylmaleic anhydride was employed as a starting

material in a sensitized [2 ỵ 2] photocycloaddition to 1,2 dichlorethene [114]. The

reaction of tetrahydrophthalic anhydride (120) with alkenols and alkynols was

conducted by direct irradiation in a Pyrex vessel. As an example, the reaction with

allyl alcohol is depicted. The exo product 121 was the preferred product with the

endo product cyclizing spontaneously to lactone 122 (Scheme 6.43) [115]. Other

alkenols reacted similarly.

In the same study, maleimides were irradiated in the presence of allyl alcohol

and allyl ethyl ether, yielding the respective cyclobutanes with significant exo

preference [115]. Diastereofacial stereocontrol was achieved in the [2 ỵ 2] photo

cycloaddition of tetrahydrophthalimide by a chiral tether. The valinol derived sub



O



hν (λ > 290 nm)

r.t. [Ph2CO]

Cl



Cl



O

O

117



78%



O



H2N

Cl



O



HN



N



HN

HN



O

HN



O



Cl

118



HN

N



HN



H 2N



O

119



Scheme 6.42



Br



Br



6.4 [2 ỵ 2] Photocycloadditon of a,b Unsaturated Carboxylic Acid Derivatives



OH

hν (λ > 290 nm)

r.t. (MeCN)



O

O



O



quant.

d.r. = 85/15



O



OH



O



O



+



O

HOOC



O



120



121



122



Scheme 6.43



Ph

O

Ph

Si

Ph

O



hν (λ > 290 nm)

r.t. (MeCN)



O



Ph

Si



O



O

O



N



86%

78% de



O



N

O



123



124



Scheme 6.44



strate 123 was transformed diastereoselectively into the product 124 of an intra

molecular [2 þ 2] photocycloaddition (Scheme 6.44) [116].

Enantioselectivity control in a [2 þ 2] photocycloaddition reaction was achieved in a

chiral, self assembled host. Fluoranthenes and N cyclohexylmaleimide underwent

an intramolecular reaction in a cage made of M6L4, with the metal M being palladium

(II) coordinated to a chiral diamine, and the ligand L being 2,4,6 tris(40 pyridyl) 1,3,5

triazine. Up to 50% ee was observed [117].

6.4.1.6 Sulfur Compounds

Sulfur analogues of the compounds discussed previously in Sections 4.1.1 4.1.5 have

been employed with some success in [2 þ 2] photocycloaddition chemistry. The

analogous nonaromatic lactones, such as 125 [118] and 126 [119], have found little

use due to the fact that the yields achieved in their cycloaddition reactions remained

low (Figure 6.2). Thiophen 2(5H) one 126 and its 5 substituted derivatives delivered

with 2,3 dimethyl 2 butene under direct irradiation conditions (l ¼ 350 nm in

cyclohexane) the corresponding cycloaddition products in yields of only 10 15%.

O

Cl

S

Cl

O



O



O

S



S



X



5



125



126



127

128



X=H

X = CN



Figure 6.2 Structures of the sulfur compounds 125 to128 employed

in [2 ỵ 2] photocycloaddition reactions.



j197



j 6 Formation of a Four Membered Ring



198



Apparently, other photochemical reactions, which occur in the singlet manifold,

are faster than intersystem crossing and compete effectively with the [2 ỵ 2]

photocycloaddition [120].

1 Thiocoumarin (127) underwent [2 ỵ 2] photocycloaddition reactions in better

yields than 126. In contrast to coumarin, cis and trans fused products are being

found, however, for example, in the reaction with 2,3 dimethyl 2 butene, possibly

because the thiopyran ring is more flexible than the pyran ring due to the longer C S

bonds. HT product is favored with electron donor substituted olefins [121]. Electron

acceptor substitution in 3 position, as in 3 cyano 1 thiocoumarin (128), leads to

an improved performance in [2 ỵ 2] photocycloaddition reactions [122].

6.4.2

Endocyclic Heteroatom Q in b-Position (Substrate Class A5)



There are two major substrate classes that fall into category A5, that is, 1,3 dioxin 4

ones (dioxenones) and 4 pyrimidinones, which will be treated in individual subsec

tions. The latter substrate class includes also the nucleobases uracil and thymine,

which are 2,4 pyrimidindiones. Sensitization by carbonyl compounds, for example,

acetone, is generally employed to promote the substrates into the excited triplet state,

which has pp character and which is responsible for [2 ỵ 2] photocycloaddition

chemistry. Alternatively, direct excitation at relatively short wavelength (l 300 nm)

may be applicable.

6.4.2.1 1,3-Dioxin-4-ones

1,3 Dioxin 4 ones represent ideal surrogates for b ketocarboxylic acids, and have

been used extensively in photochemistry and in conventional synthetic organic

chemistry. Their acetal type carbon atom at C 2 offers not only the option of easy

hydrolysis after a [2 ỵ 2] photocycloaddition, but also lends itself as a stereogenic

center, with the aid of which diastereofacial control can be achieved. It must be noted,

however, that the face selectivity achieved in the photocycloaddition of 1,3 dioxin 4

ones is opposite to the selectivity observed in the ground state [123]. An addition

occurs from the apparently more shielded face (Scheme 6.45). As an example,

the menthone derived dioxinone 129 underwent [2 ỵ 2] photocycloaddition to

cyclopentene, yielding with significant preference product 130, which results from



O



hν (λ > 290 nm)

−40 °C (MeCN)



O

O



6



129

Scheme 6.45



70%

82% de



O



H H



O

O



H

130



6.4 [2 ỵ 2] Photocycloadditon of a,b Unsaturated Carboxylic Acid Derivatives



O



O



hν (λ = 300 nm)

r.t. (EtOAc)



O

AcO



85%

d.r. = 85/15



O

O



O



H



H



O



O

+



AcO



AcO

O



O



O



O



O



O



131



j199



O



132a



132b



Scheme 6.46



an approach of the cyclopentene to the face, which seems to be shielded by the

iso propyl substituted part of the spiro cyclohexane [124].

Explanations for the outcome of the photocycloaddition reactions have been

proposed. Seebach et al. suggested that carbon atom C 6 is pyramidalized in the

excited triplet state in the opposite direction as compared to the ground state [125].

Sato proposed that different dioxinone conformations are responsible which vary

depending on the reaction type [126].

If the stereogenic center carries a tethered alkene, the diastereoface selectivity is

dictated by the direction of the tether. Diene 131 underwent a diastereoselective

[2 ỵ 2] photocycloaddition to the products 132 of a bottom approach relative to

the six membered dioxenone ring (Scheme 6.46). The simple diastereoselectivity

was not perfect, however. The endo product 132a was preferred over the exo

product 132b [127].

The deMayo type photochemistry of 1,3 dioxin 4 ones has been beautifully applied

by Winkler et al. to the synthesis of complex natural products. Substrate 133 gave

under sensitized irradiation (with acetone as cosolvent) product 134 as single

diastereoisomer (Scheme 6.47). The diastereoselectivity results from cyclic stereo

control exerted by the two stereogenic centers in the spiro bis lactone part of the

starting material. After installation of the furan, saponification and bond scission in

a retro aldol fashion generated a keto carboxylic acid, which produced the natural

product (Ỉ) saudin (135) by simultaneous formation of two acetal groups [128].

The intramolecular [2 ỵ 2] photocycloaddition of the tricyclic substrate 136, which

was employed as an epimeric mixture relative to the chlorine bearing stereogenic

center, resulted in the cyclobutane 137, which is derived from diastereoisomer 136a

(Scheme 6.48). The reaction established the intrabridgehead configuration at the

stereogenic centers C 8 and C 10 in the target compound (Ỉ) ingenol (139) [129].



O



O



O



O



O

hν (λ > 290 nm)

0 °C (MeCN/ac)



O



O



O

O



O



80%



O

133

Scheme 6.47



O



O



O

O



O

O



134



O

O



135



O

O



j 6 Formation of a Four Membered Ring



200



O



O

hν (λ > 290 nm)

0 °C (MeCN/ac)



O



O



O



O

+



O



O

60%

137/138 = 71/29



Cl



Cl



O



H



H

Cl



136a/b



137



138



HO



H



*



O

OH

OH



8

10



O



O

OH



O

Cl

H



H

139



136b (T1)



Scheme 6.48



The facial diastereoselectivity can be explained by the rigid conformation of the

starting material, which enables attack of the alkene to the chromophore only from

one direction. The chain length of the alkenyl group is responsible for the desired

endo addition mode. The unexpected formation of product 138 was explained by

a hydrogen abstraction in the excited state of diastereoisomer 136b, which is for

steric reasons not suitable for a direct photocycloaddition. The radical adjacent to

the C Cl bond is stabilized by chlorine in a bridged chloronium intermediate

which can, after hydrogen transfer to the allylic position, undergo an intramolecular

[2 ỵ 2] photocycloaddition [130].

In other recent examples, 1,3 dioxin 4 ones have been employed in synthetic

efforts to prepare solanoeclepin A and kainic acid, modifications of which were

previously mentioned (Schemes 6.31 and 6.33) [89, 91].

6.4.2.2 4-Pyrimidinones

The intramolecular photodimerization and [2 ỵ 2] photocycloaddition in DNA in

volves thymine or cytosine as the chromophore. This chemistry has been intensively

investigated with regards to DNA damage and repair [131]. Despite the fact that the

area is of continuous interest [132], the synthetic applications are limited and are not

covered here in detail. However, some preparative aspects of 4 pyrimidinone

photocycloaddition chemistry will be addressed. Aitken et al. have prepared a

plethora of constrained cyclobutane b amino acids by intra or intermolecular

[2 ỵ 2] photocycloaddition to uracil and its derivatives [133, 134]. In a chiral adap

tation of this method, the uracil derived enone 140 was employed to prepare the

diastereomeric cyclobutanes 141 in very good yield (Scheme 6.49). The compounds

are easily separated and were despite the relatively low auxiliary induced diaster

eoselectivity well suited to prepare the cis 2 aminocyclobutanecarboxylic acids 142

in enantiomerically pure form. Enantioselective access to the corresponding trans

products was feasible by epimerization in a position to the carboxyl group [135].



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