1.2- Diols epimers

From the preceding data it can be seen that reduction studies with NaBH4 have not been as extensive as those with Li AIH4. Reduction at the 4-position of the 5a-H series should give a predominance of the -epimer. Reduction of a 2-keto-3j -acetoxy compound gives 84% of the 2, 3)5-diol. ° In the 5J -H series reduction of 7-ketocholanic acid gives a 75% yield of 7a-hydroxy-cholanic acid with only traces of the 7 -ol, but 7-ketocholesterol gives a mixture of the 7a- and 7j8-epimers. ... 

The vicinal diol of the monoterpene series, (15,2S,3/ ,5S)-(+)-2,6,6-trimethylbicyclo [3.1.1]heptane-2,3-diol (1), was converted upon reaction with methyl dichloro-phosphite into a tricyclic phosphite 2 showing a 95 5 ratio of epimers differing at the phosphorus stereocentre (Scheme 1). Its complexes with Rh(I) and Pd(II) were found to have the structures (u-Cl)2[Rh(CO)L, and ris-CL,PdL2 respectively [16]. 

Conversion of the resulting separate D-seco D-E trans i-vincadiffor-mine diols 198-201 to their primary tosylates and tertiary trimethylsilyl-oxy derivatives 202-205 and coupling to vindoline by the chlorination-silver tetrafluoroborate-potassium borohydride sequence provided amino tosylates 206-209, which could be directly subjected to cyclization or, alternatively, converted to the C-20 -C-21 epoxides 178, 181, 210, and 211 by reaction with tetrabutylammonium fluoride (Scheme 53). While cyclization of the tosylates 206-209 led essentially only to quaternary salts which could be debenzylated to provide the lower energy atropi-somer of vinblastine (1), leurosidine (56), vincovaline (184), and its C-20 epimer (212) respectively, cyclization of the epoxides 178, 181, 210, and... 

In an effort to obtain a salvinorin derivative possessing an oe-diol system which can be transformed into the dibenzoate ester required for the exciton chirality CD method, salvinorin A (1) or B (2) was treated with sodium borohydride in various protic solvents. The products having the la,2a-diol group were obtained in high yield. However, this reduction was accompanied by extensive isomerization at C-8. While mechanistic details for this unexpected observation remain to be established at this time, the isomerization at C-8 appears to be the result of the base-promoted clevage of the C-8/9 bond under the reaction conditions followed by the reclosure to provide the 8-epimer prior to the reduction of the 1-ketone. Furthermore, attempts to obtain the 1,2-dibenzoate derivative of the major reduction product 3 under various benzoylating conditions invariably produced only the 2-monobenzoate. 

An alternate entry to the narciclasine class of alkaloids has provided access to compounds related to isonarciclasine (263) (Scheme 24). In the event, the aryla-tion of p-benzoquinone with diazonium salts derived from the aryl amines 250 and 251 yielded the aryl-substituted benzoquinones 252 and 253, respectively (146). The selective hydroxylation of 252 and 253 with osmium tetraoxide provided the corresponding m-diols 254 and 255. Catalytic hydrogenation of 254 and 255 using Pd/C or Raney Ni and subsequent lactonization gave the triols 256 and 257 together with small amounts of the C-2 a-epimers 258 and 259. Aminolysis of 256 and 257 afforded the corresponding racemic tetrahydrophen-anthridones 260 and 261, whereas similar treatment of the a-epimers 258 and 259 led to the formation of ( )-isolycoricidine (262) and ( )-isonarciclasine (263), respectively. 

Draw the structural formulae of all the epimers of (2.R,3S)-bicyclo[2.2.1]-heptane-2,3-diol. 

The epimers of (2 ,3 S)-bicyclo [2.2.1] heptane-2,3-diol are (2 ,3 )-bicyclo-[2.2.1]heptane-2,3-diol and (2S,3S)-bicyclo[2.2.1]heptane-2,3-diol. Note that epimers differ in the absolute configuration at only one chirality centre. As a consequence of structural constraints, only two epimers are possible since the bridge can only be formed by a cis linkage. 

The diols 4 and 6-10 are particularly suited for the preparation of small molecules, like insect pheromones, that contain relatively few chiral carbon atoms in their framework and whose chirality is due to oxygen substitution.From related isopropyli-dene structures it is possible to synthesize the chiral C, C and Cg carbonyl compounds (11)-(14) by ozonolysis, that are convertible into the a-epimers (15)-(18) by base treatment. Furthermore, the number of synthons that are potentially available is increased by the adducts accessible from (11)-(18) by reaction with suitable carbon nucleophiles under conditions allowing control of the stereochemistry by the two oxygen functions. 

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