High-and atmospheric-pressure cycloaddition of trichloroacetyl isocyanate (2) to 3,4-di-&acetyl-L-rhamnal (11) leads to formation of four products: a-gluco (k2)cycloadduct l2, two stereoisomeric a-gluco 13 and B-manno 14 (2+2)cycloadducta , and o,&unsaturated amide 5. The isocyanate enters the rhamnal molecule preferentially anti with respect to the 3-O-acetyl substituent. It is found that under conditions of both -high and normal pressure (4+2)cycloaddition is preferred over (2+2)cycloaddition, more so under high pressure. Under normal pressure, better stereoselectivity in (2+2) cycloaddition is obtained. Elevation of temperature and prolongation of the reaction time promote rearrangement to a,&unsaturated amide. Transfor~tions of the reaction products into N-unsubstituted: l-O-trichloroacetyl compound 20, methyl glycosides 23 and 24, S-lactam 2S, a,&unsaturated amide 26, and into the 4-aza-2,10-dioxabicyclo(4.2.0)decan-S-one skeleton X-E are described. Chitwood, Gott, and Martin1 have found that dihydro-2H-pyran (1) treated with trichloroacetyl isocyanate (3 affords the amide 5, via intermediate formation of unstable B-lactam 1, and (4+2)adduct 4. They have observed that the reaction performed in acetonitrile proceeds ten times faster than that run in chloroform. Owing to their low stability, both by-products have only been detected in a n.m.r. teat-tube' (Scheme 1). Scheme 1