Photocycloaddition esters

Alcohol (34) is needed and we shall want to use a photocycloaddition to make some derivative of it (ether or ester). One partner in the cycloadd ition will be nucleophilic (CH2=CH0R) so we need a carbonyl group conjugated with the cyclohexene. FGA is required. 

There are ample evidences which suggest that a cyclic periodate ester is the intermediate 148). Cyclobutane-1,2-diols can also be cleaved oxidatively and this aspect has been used in organic synthesis. Thus, photocycloaddition of l,2-bis(tri-methylsiloxy)cyclobutene to (—)piperitone (381) gave the photoadduct (438). Desilyla-... 

Other five-membered heterocyclic enones used in [2 + 2] photocycloadditions are 1//-pyrrole-2,3-diones86-88 and derivatives of maleic anhydride. Some cycloadducts of dimethylmaleic anhydride and unsaturated acid esters have been used as monomers for condensation polymers.89... 

A novel entry to decahydrocyclopentacyclooctene derivatives via the intramolecular photocycloaddition of fused a,/3-unsaturated y-lactones has been developed (80CC1011). Irradiation of the butenolide (153) in acetone solution gave both the fused and bridged photoadducts (154) and (155) (2-3 1). The major adduct was hydrolyzed, oxidized and esterified to afford (156). Reductive cleavage of the unsaturated keto ester (156) with lithium in ammonia afforded a five-component mixture of a,/3- and /3,y-unsaturated esters. Equilibration with 0.1M sodium methoxide in methanol converted the mixture into a single a,j8-unsaturated ester (157 Scheme 34). This annelative two-carbon ring expansion method may find application in the synthesis of ophiobolin and ceroplastol sesterterpenes. 

Sensitized photocycloaddition of thiophene and 2,5-dimethylthiophene to acetylenedicar-boxylic ester leads to the phthalic esters (303) (73CB674). In the mechanism suggested, the triplet state of the thiophene adds to the alkyne to form a biradical, which cyclizes to the... 

Scheme 30 Intramolecular ortho photocycloaddition of ketones, nitriles, and esters.

Scheme 58 Intramolecular ortho photocycloaddition of 3-alkenyloxy-5-hydroxy- and 3-alkenyloxy-5-aIkoxybenzoic acid methyl esters.

Gilbert and his co-workers reported that the photocycloaddition of 1-cyanonaphthalenes (260 and 262) gave 261 and 263, independently [288] (Scheme 72). In addition, they prepared and photoirradiated a variety of compounds that connected naphthalene and alkenes by methylene, ether, and ester tethers at various lengths [289], Among them, the reaction of 264 gave the best result, whereas the 2-substituted derivative (265) did not give any product. 

Diastereoselectivity in the (3 + 2) photocycloaddition of di-( )-menthyl, di-( )-8-phenylmenthyl, and di-( )-bomyl derivatives (398b-d) is largely dependent on the concavity of the auxiliary, steric bulk of the substituents of the alkenes, and reaction temperature [365] (Scheme 107). The highest de value was obtained when 8-phenylmenthyl ester 398c was employed, although the yield of the adducts was relatively low. The bomyl ester 398d showed poor diastereoselectivity. The effect of the chiral auxiliary seems to be reasonably explained in terms of the concavity of the auxiliary. When a-methylstyrene was used as the alkene, two stereoisomers of (3 + 2) adducts were obtained in lower yields than... 

Somekawa, K. Okuhira, H. Sendayama, M. Suishu, T. Shimo, T. Intramolecular [2 + 2] photocycloadditions of l-(cu-alkenyl)-2-pyri-dones possessing an ester group on the olefmic carbon chain. J. Org. Chem. 1992, 57, 5708-5712. 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

Control over the absolute configuration in cyclohexenone photocycloadditions has been achieved by auxiliary-induced diastereoselectivity. In particular, esters related to compound 26, which are derived from a chiral alcohol but not from methanol, lend themselves as potential precursors, from which the chiral auxiliary can be effectively cleaved [42, 43]. In a recent study, the use of additives was advertised to increase the diastereomeric excess in these reactions [44], An intriguing auxiliary-induced approach was presented by Piva et al., who employed chiral 13-hydroxy-carboxylic adds as tethers to control both the regioselectivity and the diastereoselectivity of intramolecular [2 + 2]-photocycloaddition reactions [45]. In Scheme 6.14 the reaction of the (S)-mandelic acid derived substrate 38 is depicted, which led with very good stereocontrol almost exclusively to product 39a, with the other diastereoisomer 39b being formed only in minor quantities (39a/39b = 96/4). Other acids, such as (S)-lactic acid, performed equally well. The chiral tether could be cleaved under basic conditions to afford enantiomerically pure cydobutane lactones in good yields. 

Photocycloaddition ofVinylogous Amides and Esters (Substrate Classes A2 and A3)... 

Griesbeck, A.G., Bonduck, S., and Lex, J. (2003) Synthesis of erythro-a-amino [1-hydroxy carboxylic acid esters by dias-tereoselective photocycloaddition of 5-me-thoxyoxazoles with aldehydes. Journal of Organic Chemistry, 68 (26), 9899-9906. 

The first report concerning an asymmetric Paterno-Buchi reaction with a chiral carbonyl component was reported in 1979 by Gotthardt and Lenz [128]. The photocycloaddition of the enantiomerically pure menthyl ester of phenylglyoxylic acid 133 with 2,3-dimethyl-2-butene gave the oxetane 134 with a diastereomeric excess of only 37% (Sch. 45). 

Until now, only few attempts were made to control the absolute configuration of the chiral polycyclic products. When salicylic esters of type 77 (Sch. 14) possessing an asymmetric alcohol moiety as chiral auxiliary instead of the n-butyl group are irradiated, the corresponding products can be isolated with 17% ee [50]. Better results can be obtained with the additionally acylated substrates 79a-c (Sch. 15). Compounds 80a-c were isolated with diastereomeric excesses up to 90% [29,56]. These compounds were formed via [2+2] photocycloaddition yielding the intermediates X. A thermal rearrangement leads to 80a-c. Compounds 81a-c are produced in the same time via a further photochemical rearrangement but due to thermal reversibility of the last step, 80a-d could be isolated with yields up to 90% when the reaction was completed. 

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