DIASTEREOSELECTIVE PHOTOOXYGENATION OF CHIRAL NAPHTHYL ALCOHOLS - THE HYDROXY GROUP DIRECTING EFFECT IN SINGLET OXYGEN [4+2] CYCLOADDITION TO ARENES

摘要

A series of chiral naphthyl alcohol derivatives 1 was prepared and submitted to sensitized photooxygenation. The corresponding endoperoxides 2 were formed in high yields through [4 + 2] cycloaddition of singlet oxygen. The pi-facial selectivity of singlet oxygen attack was determined and the stereochemistry of the product assigned for representative cases (1a,h) by X-ray analysis of the triols 3 derived from the endoperoxides by reduction. In the photooxygenation of the alcohols 1a-g in nonpolar solvents, the (alpha R*,1R*,4S*)-configurated endoperoxides 2a-g were formed preferentially (diastereomeric ratio (dr) greater than or equal to 85:15). Increase in solvent polarity or protection of the hydroxy group as the acetate in 1i or as the silyl ether in 1j led to substantial loss of diastereoselectivity. Placement of a methyl group at C2, as in alcohol 1h, gave high but opposite pi-facile selectivity (dr = 94:6), i.e., (alpha R*,1S*,4R*)-2h was formed as major product. The observed substitution effects on the pi-facial selectivity are rationalized in terms of steric and electronic control. Thus, hydrogen bonding operates between the unprotected hydroxy group and the incoming singlet oxygen dienophile. Peri strain leads to an effective energy discrimination of the respective diastereomeric transition states and, consequently, high pi-facial selectivities. An alkyl substituent at C2, however, induces additional ortho strain of 1,3-allylic origin, which overrides the effect of peri strain to afford highly selectively the opposite stereoisomer. [References: 26]