Diastereoselectivity facial selectivity, chiral alkenes

A few recent studies have described the AA of substrates containing chiral [40, 70, 72, 94, 97, 98] or prochiral [99-102] centers. In the AA of chiral substrates, double diastereoselectivity arose from the interaction of the substrate with the chiral ligand. The AA proceeded with the sense of facial selectivity expected for the DHQD or DHQ ligands. The effect of the chiral center on the facial selectivity has not been investigated. A study of the AA of the chiral alkene 79 with the pseudoenantiomeric forms of the PHAL ligands revealed that the DHQ and the DHQD ligands led to matched (70% de) and mismatched (29% de) reactions, respectively (Scheme 18) [97]. 

Ketals 201 from cyclopentenone and dialkyl tartrates have a rigid cyclopentene ring. Submitted to 2 -i- 2 photocycloaddition with a cyc-lohexenone and a cyclopentenone carboxylic ester, diastereoselectivities of up to 84% were obtained. The selectivity was very sensitive to the steric hindrance of the chiral auxiliary and tartrate derivatives gave better results than the corresponding threitoldibenzylethers [162]. When the chiral auxiliary was introduced into an acylic enone system (204), the diastereoselec-tivity of the cycloaddition was low. This indicates that the chiral alkene in the ground state exists as a mixture of conformers leading to opposite facial diastereoselection [163]. 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

The facial diastereoselectivity derived from-the ratio (3 + 4)/(5 + 6) was 50%, while the exo/endo selectivity derived from the product ratio (3 + 5)/(4 4- 6) was 40%. Oxetanes 9a,b were obtained with a low diastereoselectivity from the reaction of (R)-isopropylideneglyceraldehyde 7 with 3,4-dimethylfuran 8 [6]. Oxetanes 9a,b have been used for the synthesis of asteltoxin. Enantiopure acyl cyanides were used in the same way as chiral carbonyl reaction partners [7] and camphor for the addition with electron-poor alkenes like dicyanoethylene [8]. In the latter case the reaction occurs in the S i state of the carbonyl compound. 

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