By nucleophilic attack onto carbonyl groups

Piperidines bearing a masked aldehyde function in the e-position are easily transformed into quinolizidine compounds through intramolecular reductive amination after deprotection (acetal precursors) or oxidative cleavage (tv e-diols). Some examples are given below. 

An enantioselective synthesis of (—)-lupinine 6 was based on a similar reductive amination process. In this case, (k)-phcnylglycinol was used to obtain a chiral nonracemic oxazololactam which was cyclized after reduction of N-C and O-C bonds and subsequent hydrolysis of the masked aldehyde 2004T5433 . 

Many polyhydroxylated quinolizidines 1995CRV1677 , frequently designed as azasugars, are powerful glycosidase inhibitors and therefore have potential therapeutic application. The 7-oxa-l-azabicyclo[2.2.1]heptane derivative 191, obtained from 3-0-benzyl-l,2-0-isopropylidene-l,5-pentadialdo-a-D-xylofuranose with 

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