STEREOCHEMISTRY OF 1,2-OXAPHOSPHOLANES. IX. CONFORMATIONAL BEHAVIOR OF SUBSTITUTED 2-METHOXY-2-OXO-1,2-OXAPHOSPHOLAN-3-OLS AND RELATED COMPOUNDS

Abstract

Conformational behavior of the 1,2-oxaphospholane ring is discussed in terms of pseudorotation using the Altona-Sundaralingam approach. From the known angular dependences of vicinal H—C—C—H, H—C—C—P, H—C—O—P, C—C—C—P and C—C—C—P coupling constants, “the best” Karpluslike relationships are selected and they are employed in establishing vicinal coupling constant vs. angle of pseudorotation relationships for use in the conformational analysis of the 1,2-oxaphospholane ring. Based on the available vicinal coupling constants, conformational preferences for 33 compounds are discussed. Some of them exist in well-defined single conformations, others as equilibrating mixtures of different forms. In a few instances, however, the spectroscopic data could not be interpreted unequivocally. Pseudoequatorial orientation of C-substituents (alkyl, aryl) attached to carbon atoms is a primary factor governing conformations of the 1,2-oxaphospholanes studied. It is not clear whether the orientation of substituents at phosphorus (anomeric effect) has a significant influence on the conformation of the ring in diastereoisomeric pairs. Gauche effects are found to operate in some cases.

Key Words:

• 1,2-Oxaphospholanes
• conformational analysis
• pseudorotation
• ¹H NMR
• ¹³C NMR
• vicinal coupling constants