Synthesis and hydrolysis of optically active naphthyl-phenyl-bis(acyloxy)spiro-λ 4-sulfane: absolute configurations of spiro-λ 4-sulfanes, related sulfonium salts and naphthyl phenyl sulfoxides determined by CD spectroscopy using exciton chirality and empirical rules

Abstract

( S)-(+)- and ( R)-(−)-enantiomers of naphthyl-phenyl-bis(acyloxy)spiro-λ 4-sulfane 2 were prepared from the precursors ( R)-(−)- and ( S)-(+)-naphthyl phenyl sulfoxide dicarboxylic acid 2a with acetic anhydride. The hydrolysis of spiro-λ 4-sulfane ( R)-(−)- 2 in acidic and basic solutions gave as major products sulfoxide ( S)-(+)- 2a and ( R)-(−)- 2a, respectively. The selectivity of the reactions was interpreted with the different reactivities of the ortho and peri neighboring carboxylic groups of the sulfoxide 2a, and with the different ability of the five and six-membered rings to be cleaved in spiro-λ 4-sulfane 2. The absolute configurations of the enantiomeric spiro-λ 4-sulfanes 2 and sulfoxides 2a were determined by CD spectroscopy. Exciton coupling was observed in the spectrum of spiro-λ 4-sulfane 2 between the benzene 1L a and naphthalene 1B b transitions, which results in a negative couplet for ( R)-(−)- 2. Exciton coupling can be expected only for the 1B b transitions of the S-phenyl and S-naphthyl rings of the sulfoxides, which define a positive couplet for ( S)-(+)- 2a. Empirical rules were found to be valid for the determination of configuration of spiro-λ 4-sulfanes and related sulfonium compound derivatives with a trigonal bipyramidal structure, and for that of naphthyl-phenyl-sulfoxides. Both spiro-λ 4-sulfanes and sulfoxides should be viewed from the side of the lone pair; the axial substituents in the former case and the downwards oriented SO bond in the latter case are placed in a vertical plane. Rings and substituents in the upper-right and lower-left sectors define positive exciton couplets, while the opposite case is characterized by negative exciton-coupled circular dichroism (EC-CD).