Isopropylidene ring cleavage

The Mass Spectra of O-Isopropylidene Ketals 9 (Figure 6), 10, (Figure 7), and 11 (Figure 8). Three fragmentations characteristic of O-isopropylidene ketals are loss of a methyl radical from the ketal ring, cleavage of bonds adjacent to the ketal ring, and loss of acetone (12). 

Table I. Ring Cleavage of 5,6-Anhydro-isopropylidene-D-glucof uranose.. 68...

The reaction of 2,2,4,4-tetramethyl-l,3-cyclobutanedione with tetraphosphorus decasulfide (P4S10) led to ring cleavage and the formation of the intermediate dimethylthioketene, which underwent [2+2] cycloaddition after 12 h in refluxing pyridine to give 4-isopropylidene-3,3-dimethylthietane-2-thione 88 in nearly quantitative yield (Equation 28) <2004BCJ187>. 

An interesting contrast has been observed between the respective actions of acid and alkali on 3,4-anhydro-l,2-isopropylidene-D-tagatose (XXIVo). The anhydro ring of this compound shows considerable resistance to both mineral acids and alkali. Cleavage by sodium meth-... 

The third adduct (669) was an ether after similar functionalization of the dihydropyran ring to the L-gulo system, 3-0-(benzyl 4-deoxy-/3-L-gulopyranosid-4-yl)- l,2 5,6-di-0- isopropylidene-a-D- glucofuranose (672) was obtained. Cleavage of the ether bond between the sugar units was possible35 by alkaline -elimination. 

The dithiane derivative 60 (Scheme 14) is such a compound, it being made from 2,3 5,6-di-0-isopropylidene-D-mannose by treatment with 2-lithio-l,3-dithiane to give a heptose dithioacetal that was refunctionalized at C-2-C-3 by way of the C-l anion and then converted to the 6,7-epoxide following selective acid-catalyzed cleavage of the 6,7-acetal ring. Treated with n-butyllithium it gives, in 70% yield, the cyclized 61, which is efficiently convertible into validatol 62, a component of validamycin A, by desulfurization with Raney nickel and de-O-protection by use of boron tribromide in dichloromethane [31]. 

The synthesis of the /-hydroxy lactol ring starts with the cleavage of the PMB protecting group and the oxidation of the resulting hydroxy compound to the enone 22 (Figure 7) [20], Acid-induced removal of the isopropylidene group results in the spontaneous formation of... 

---