article pyran derivative alkadiene cycloaddition enantiomer priority journal stereochemistry unclassified drug 1,3 butadiene derivative Betulaceae Diels Alder reaction Lewis acid Stereoisomerism Synthesis (chemical) catalysis cyclopentadiene derivative diastereoisomer proton nuclear magnetic resonance pyranoside synthesis 1,3 butadiene 1,3 cyclohexadiene 2 acetoxy 3,4 di o acetyl dextro arabinal 2 benzyloxy 2h pyran 3(6h) one 2 propyl 2,6 diamino 2,3,4,6 tetradeoxy alpha dextro threo hexopyranoside 2 propyl 2,6 n,n diacetamido 2,3,4,6 tetradeoxy alpha dextro threo hexopyranoside 2,3 dimethylbutadiene 6 acetoxymethyl 2 (2 propyloxy) 2h pyran 3(6h) one 6 methyl 2 (2 propyloxy) 2h pyran 3(6h) one Acetylation