A highly stereoselective approach to the synthesis of functionalized pyran derivatives by Lewis acid assisted ketal reduction and allylation.
Reduction of bicyclic ketal 1 gave functionalized pyran derivatives 7a or 7b in a highly stereoselective manner, depending upon the reduction conditions utilized. For example, treatment of ketal 1 with TiCl4/Et3SiH produced exclusively diol 7b with the 2,5-syn relationship in good yield. Alternatively, reduction of ketal 1 by DIBALH gave 2,5-anti-diol 7a stereoselectively. Alane reductions of ketal 1 were highly stereoselective also; however, the syn/anti selectivity observed was strongly dependent on the ratio of reagents employed for in situ generation of the alane. Lewis acid catalyzed allylation of ketal 1 gave pyran 10 in a stereospecific alkylation reaction.
Bogaczyk
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S
Department of Chemistry and Biochemistry, The University of Maryland, College Park, Maryland 20742, USA.
Brescia
M R
MR
Shimshock
Y C
YC
DeShong
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P