In preceding articles',2, we have described stereoselective syntheses of nitropyrazolines by 1,3-dipolar cycloaddition of diazoalkanes to sugar nitro-olefins having the n-galacto and D-manno configurations. We now report the preparation of two new sugar nitro-olefins (5 and 6) bearing bromine as another functional group attached to the a-position, and their reactions with diazoalkanes .
These compounds were prepared from (E)-3,4,5,6,7-penta-O-acetyl-1,2-dideoxy-1-C-nitro-D-galacto(and -D-manno)-hept-l-enitol (1 and 2)3°4 by bromination and dehydrobromination of the resultant vic-dibromides . The addition of bromine to nitro-olefins in nonpolar solvents can be the result of a competition between Ad E 2 and Add mechanisms 5. Both are stereospecific reactions and only two of the four possible isomers are formed. Thus, in the bromination of 1 and 2, crystalline solids were isolated, in high yields, and identified as mixtures of the two diastereomeric 1,2-dibromo-1,2-dideoxy-2-C-nitroheptitols (3,3', 85 : 15 ratio; and 4,4', 94 : 6 ratio), which could not be resolved chromatographically . The 1,2-elimination of hydrogen bromide from the mixture of dibromides gave the bromonitro-olefins 5 and 6 . The conditions employed must promote a syn-elimination 5 to give the Z-olefins . This configuration is in agreement with previous results on this reaction' and could be inferred from the chemical shifts of the olefinic protons (7 .33 and 7.44 ppm).
The favored conformation of 5, in solution, must be similar to that shown for other sugar olefins having D-galacto" 7 and D-manno 2 configurations previously described, in which H-2 and H-3 have an anti disposition (J2.3 7.9 Hz), H-3 is eclipsed with the ethylenic bond, and no 1,3-parallel interactions exist among the acetoxyl groups (Fig. 1). In this conformation, the lsi,2re face of the nitro-olefin is sterically hindered by the sugar chain . A similar conformation is also proposed for * Corresponding author.