Diastereoselective medium-sized ring

The stereoselective of the ring expansion reaction for the nonracemic 2-pyrrolidine acrylic ester 246 and related systems has been thoroughly investigated by Nubbemeyer. The reaction proceeds via the chair-like transition state 247 and yields 2-azonine 248 with excellent trans-diastereoselection. Products characteristically display E-olefin embedded within a medium-sized ring. Nubbemeyer has studied the relative stability of these slowly interconverting conformers of medium-ring lactams which exhibit planar chirality and effectively demonstrated this chemistry in natural product synthesis. ... 

The acetal [1,2]-Wittig rearrangement protocol is also applicable to the synthesis of medium-sized cyclic ethers. For example, a reaction of the 9-membered cyclic acetal 37 with lithium piperidide provides the 8-membered ring ether 38 in good yield along with high diastereoselectivity (equation 20) . 

A diastereoselective photochromism of a bisbenzothienylethene was reported by Yokoyama et al. By employing A1,3-strain around the double bond of one of the benzothiophene of D-9, the conformation of the stereogenic carbon atom with regard to the hexafluorocyclopentene ring was fixed. As a result, the second benzothiophene lay closer to the medium-sized substituent so that it would generate one diastereomer predominantly by photoirradiation. Indeed, the diastereosel-ectivity of photocyclization of D-9 in toluene at room temperature was 88% de at 85% conversion [41]. 

Scheme 6.7. Diastereoselectivity in conjugate addition of organocuprates to chiral cyclic enones of medium ring size.

The study of chiral 1,4-threitol-derived dioxolanes was also extended to a series of medium and large ring olefins. Very high diastereoselectivities were obtained for both (Z)- and ( -configurated olefins in ring sizes of five to sixteen63. 

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