In search of open-chain 1,3-stereocontrol

Abstract

Methylation of methyl 4-phenylpentanoate 25 gives the diastereoisomers methyl (2RS,4SR )-2-methyl-4-phenylpentanoate 26 and methyl (2RS,4RS)- 2-methyl-4-phenylpentanoate 27 in a ratio of 44∶56. The aldehydes 3-dimethyl(phenyl)silylbutanal 28, 3-dimethyl(phenyl)silyl-3-phenylpropanal 32 and 3-dimethyl(phenyl)silyl-4-methylpentanal 36, each of which has a stereogenic centre on C-3 carrying a silyl group and successively also a methyl, a phenyl and an isopropyl group, react with a range of methyl, phenyl and isopropyl nucleophiles to give pairs of diastereoisomeric secondary alcohols 40–42, 47–49 and 54–56 having 1,3 related stereocentres. The same alcohols 43–45, 50–52 and 57–59 are also prepared by reduction of the corresponding ketones 29–31, 33–35 and 37–39 with a range of hydride reagents, and three of the ketones, 31, 35 and 39, react with phenyllithium to give mixtures of the tertiary alcohols 46, 53 and 60. The (E)- and (Z)-α,β-unsaturated methyl esters, 61, 62, 64, 65, 67 and 68, prepared from the same three aldehydes with methoxycarbonylmethyltriphenylphosphorane, react with the phenyldimethylsilyl-cuprate and -zincate reagents to give diastereoisomeric pairs of 1,3-disilylated esters. Likewise, the α,β-unsaturated dimethyl diesters, 63, 66 and 69, prepared from the same three aldehydes with dimethyl malonate, react with phenyldimethylsilyllithium and the corresponding cuprate and zincate reagents to give diastereoisomeric pairs of 1,3-disilylated diesters, and with various methyl and phenyl nucleophiles to give the corresponding pairs of diastereoisomeric diesters with stereogenic centres at C-3 and C-5. The relative stereochemistry of all but two of the products having 1,3-related stereocentres has been proved by silyl-to-hydroxy conversion using mercuric acetate and peracetic acid to give the corresponding alcohols or their derived lactones.An attempt to identify a purely steric rule by which it might be possible to predict which diastereoisomer would be the major product in each of these reactions was based on arguments about, and molecular mechanics calculations of, the lowest-energy conformations of the starting materials. The only rule that emerges is that ketones are regularly attacked in sense B, defined in the drawings 21 and 23, in a conformation that minimises the interaction between the group M on the stereogenic centre and the group R¹ on the other side of the ketone, but even within this group of reactions, phenyl groups in either or both locations sometimes lead to anomalies.Krapcho reactions take place more rapidly and in higher yield using four equivalents of lithium chloride in place of the usual sodium chloride, and adding two equivalents of water to the DMSO.