Pyridines. Partie IX‐Pluribenzylation de La S‐Collidine et de La Lutidine‐2,6

Abstract

AbstractA literature survey of the polymetalation of 2,6‐lutidine 2 shows different results about the site(s) of deprotonation and condensation with various electrophiles. This discrepancy prompts us to examine the polylithiation (PhLi 3 equivalents, Et2O) of s‐collidine 1, 2,6‐lutidine 2 and more briefly of 2,4‐lutidine 3. Polylithiation takes place exclusively at the methyl groups of the same position vs the ring nitrogen: 2‐and/or 6‐methyl of 1. The previous regioselectivity observed in the case of the monometalation in Et2O is maintained. From 1 and 2, polylithiation gives picolyl anions which lead by reaction with PhCH2Cl, simultaneously to mono‐, di‐symmetrical, di‐non‐symmetrical α‐derivatives, and tri‐and tetra‐symmetrical α,α'‐compounds; but no product is formed from a benzylic β‐anion. In the same way, the deprotonation of 2‐methyl‐6‐(2‐phenylethyl) pyridine 17 leads to 2,6‐bis (2‐phenylethyl) pyridine 19 and 2‐methyl‐6‐dibenzylmethyl pyridine 18, to the exclusion of the β‐compound 2‐methyl‐6‐(2,3‐diphenylpropyl) pyridine 21. The latter compound is formed in low yield, besides 19 and 18, in the conditions of the Tchitchibabine reaction (NaNH2, toluene). The structure of the benzylated compounds was established mainly from the 1H‐nmr data. No product was isolated which would result from the nucleophilic attack of the generated carbanions at the pyridine carbons.