Spirobinaphthopyrans: Synthesis, X-ray crystal structure, separation of enantiomers, and barriers to thermal racemization

Chiral spirobinaphtopyrans 1-9 were synthesised using the acid catalyzed al dol condensation of 2-hydroxy-1-naphthaldehyde or 1-hydroxy-2-naphthaldehyd e with the appropriate ketone and subsequent reaction of the isolated pyryl ium salt with base. Separation or enrichment of enantiomers was accomplishe d by low-pressure liquid chromatography (LPLC) on triacetylcellulose, tribe nzoylcellulose, or (+)-poly(triphenylmethylmethacrylate) on SiO2. The barri ers to thermal racemization were determined by dynamic H-1 NMR measurements and by on-line or off-line measurements of the enriched enantiomers after LPLC. Barriers to thermal racemization for reversible cleavage of the C-spi ro-O bond in 1-9 were found to be in the range 85-105 kJ/mol and are ration alized by steric and electronic effects. The Delta G(not equal) values decr ease with decreasing length of the bridge or decreasing size of substituent s in the 3 and 3' positions. Therefore, Delta G(not equal) for 7>9>6, 3>2, and 4>1. Geometrical data from X-ray crystal structure analysis show that t he mean value of the C-spiro-O bond length in the spirobibenzopyran 14 is s horter, i.e., the bond is stronger than that in the spirobinaphtho derivati ves 4, 3, and 6. This bond-length shortening is explained by the weaker con jugation of the oxygen pi-lone electron pair with the pi-system of the benz ene ring in 14 compared to the naphthalene moiety in 3, 4, and 6, Chirality 11:363-372, 1999. (C) 1999 Wiley-Liss, Inc.