Addition of Trifluoroborates to Oxetanyl N,O-Acetals: Entry into Spiro and Fused Saturated Heterocycles

N,O-Acetals derived from 3-oxetanone and 1,2-amino alcohols undergo addition reactions with alkynyl-, allyl-, allenyl-, and vinylpotassium trifluoroborates. The resulting products undergo cyclization by orthogonal activation of the oxetane and the alkyne to give a diverse set of spiro and fused heterocycles.