solvent peak was used as an internal reference (6H 7 20).J &-0.3 Hz; only geminal and vicinal coupling constants are recorded. Relaxation times, TI, k 0 . 0 3 s ; values in brackets are probably less accurate because t h c signal had others overlapping. 1; Not meawred. II Detcrmincrl from 400 MHz lH spectrum of deuteriated cobester (8). * With greater resolution enhancement, this signal appeared as a doublet of doublets (see Table 1).

When cobyrinic acid is biosynthesised from uro'gen-Ill in deuterium oxide, deuterium is inserted at both C-18 and C-19 as shown by 13C n.m.r.; this is a further result in keeping with a biosynthetic sequence for the corrin ring of vitamin Ell2 involving no external redox reagents.

The reaction of pyridinio-iV-phosphonates with pyridines in aqueous buffers has been demonstrated to involve nucleophilic attack at phosphorus. The second-order rate constants obey the equation log kxpy

No abstract

Die Reaktion der Acetale (I) mit den Nucleophilen (II) und (III) ergibt die Aldolether (IV)/(V) in hoher Ausb. und Diastereoselektivität. Umsetzung mit Trimethylsilylbutanon statt mit (III) verringert die Ausb. erheblich.