Diols fragmentation

Yoshikoshi recently reported a 1,3-diol fragmentation (Scheme 14a) under mild conditions, where a benzylic hydroxy group activated by the 2,6-dimethoxy-4-methylphenyl group proved to be a remarkably reactive nucleofuge. 

The cyclic sulfates derived from 1,3- and 1,4-diols showed a different fragmentation pattern. For example, sulfates of 1,3-diols fragment in an orderly manner via (M-SOa ), which are represented by oxetane radical cations, whereas 1,4-diol cyclic sulfates fragmented via tetrahydrofuran-like intermediates (77AJC569). 

A. Dondoni, G. Fantin, M. Fogagnolo, A. Medici, and P. Pedrini, Hydroxy group inversion in thiazole polyols by an oxidation-reduction sequence. An entry to syn 1,2-diol fragments in masked carbohydrates, J. Org. Chem. 54 702 (1989). 

Most carbohydrates contain ethane-1,2-diol fragments, therefore they were also successfully included into crown ether macrocycles [313]. Polyvalency of carbohydrates was studied based on dendrimeric structures which were generated on successful glycosylations of highly branched polyethylene glycol units [314]. 

In contrast to the other aldoses, the copper atoms find an especially favorable coordination environment for lyxose and mannose. Cu(II), indeed, has the ability to build stable complexes with the pyranoid trans-daoX fragments, shown for the complexed 3,4-diol functions, but the more flat chelate five-membered rings, which are built with the with pyranoid m-diol fragments, seemed to be more favorable for copper centers. 

One of the key steps during the first total synthesis of (-)-aspinolide B by A. de Meijere and co-workers was the NHK reaction to form the ten membered lactone ring. The precursor for this key macrocyclization step was prepared by forming an ester from a three-carbon monoprotected diol fragment and a seven-carbon vinyl iodide fragment. Deprotection of the primary alcohol and its subsequent oxidation afforded the desired vinyl iodide aldehyde precursor. Exposure of this precursor to 15 equivalents of CrCl2 doped with 0.5% of NiCb at high dilution in DMF afforded the desired diastereomer in a 1.5 1 ratio. 

The studies of Ban and Wakamatsu culminated in the preparation of three natural compounds from a single synthetic route (Scheme 1.15). The enediol bis silyl ether 63 was converted to the dianion and immediately alkylated with l-iodo-3-butanol to give glycol 64 as a mixture of diastereomers in 87% yield. Diol fragmentation with lead tetraacetate afforded keto lactone 65 in quantitative yield. Formation of the dithioketal and subsequent Raney nickel desulfurization then gave 66 (81%). Macrocyclic lactone 66 is the simple natural product... 

Fragmentations are reactions in which the molecule breaks open by the cleavage of a C—C single bond, and we start this section with some examples. Both diastereoisomers of this cyclic diol fragment in acid to give an aldehyde. 

The use of ToF-SIMS on these polymers allowed the observation of ions with masses of over 1,000 amu. Ions below m/z 300 were derived from fragments of either the DETOSU or the diol units. Typical ions diagnostic of the DETOSU functionality are given below. The positive charge centre in all these ions can be distributed to one of two oxygen heteroatoms, only one canonical form is showm in the structures. For the HD and t-CDM polymers, the characteristic low mass diol fragments w ere found to be [R-H]+, [R+OH]+ and [R+OH3] which were obsert ed in relatively low intensity, except for the t-CDM [R-H] ion at m/z 109. It should be noted that the [R-i-OH] ion of t-CDM appears at m/z 127 and is thus obscured by one of the DETOSU peaks. 

---