Directed aldo, reaction

By an enzyme-assisted short synthesis, several tetrahydroxylated pyrrolizidines (3-e/u-australine, australine, 7-e/ /-alexine) have been directly obtained by applying an enzyme-catalysed aldo-reaction, bypassing the necessity for tedious efforts in the protecting OH-groups as demonstrated by Romero and Wong [149] (Fig. 24). 

Tlie conditions needed for aldol dehydration are often only a bit more vigorous (slightly higher temperature, for instance.) than the conditions needed for the aldo) formation itself. As a result, coiyugated cnones are often obtained directly from aldol reactions, and the iotermediate -hydroxy carbonyl compounds are not isolated. 

Aldos-2-ulose 1-phenylhydrazones react with differently substituted hydrazines to yield mixed arylosazones.347 352 In addition, they react with o-phenylenediamine to give quinoxalines (as seen in Scheme 29).347-352 It seems that the phenylhydrazone residue undergoes nucleophilic substitution with o-phenylenediamine, either directly or after hydrolysis to the al-dosulose, to give a quinoxaline lacking a hydrazone residue. If the hydra-zone residue is stabilized by chelation, as in the case of compound 175, the reaction proceeds with retention of the hydrazone residue in the quinoxaline formed (176) (Scheme 38).314... 

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