1,2,6-Triols 8-lactone synthesis

Thus, 1.7-octadiene (79), which was subjected to monohydroboration followed by asymmetric dihydroxylation of the remaining double bond to give triol 80 with approximately 80% ee. Further transformations then afforded the desired butenolide 81. Double asymmetric dihydroxylation of diene 83 and subsequent protection gave hydroxy lactone 84 [98], which was then converted into acetylenic bis(hydroxy)bistetrahydrofuran 82 as the required intermediate for the (+)-asimicin synthesis. Mitsunobu inversion at C-24 gave rise to the diastereomeric (+)-bullatacin precursor. 

Mitsunobu reaction as well as by mesylation and subsequent base treatment failed, the secondary alcohol was inverted by oxidation with pyridinium dichromate and successive reduction with sodium borohydride. The inverted alcohol 454 was protected as an acetate and the acetonide was removed by acid treatment to enable conformational flexibility. Persilylation of triol 455 was succeeded by acetate cleavage with guanidine. Alcohol 456 was deprotonated to assist lactonization. Mild and short treatment with aqueous hydrogen fluoride allowed selective cleavage of the secondary silyl ether. Dehydration of the alcohol 457 was achieved by Tshugaejf vesLCtion. The final steps toward corianin (21) were deprotection of the tertiary alcohols of 458 and epoxidation with peracid. This alternative corianin synthesis needed 34 steps in 0.13% overall yield. 

D-Xylose is converted in three steps into the 2,5-0-diacetyl 3-deoxy-D-erythropen-tono-1,4-lactone 1, a versatile intermediate towards a series of 1,2,4-triol derivatives 2-6 [41 4]. These compounds are precursors in the synthesis of a fluoro analog of nucleoside (F-ddC from 2) [41] and of avocadotriol derivatives (from 4 and 5) [43] (Scheme 3). 

The MTM group was used to protect several of the alcohol groups in Corey s synthesis of erythronolide A.31 Alcohol 47 contains a sensitive OH group, and it was converted to the MTM derivative (48). This allowed the construction of the tris-MTM lactone (49). Treatment with potassium carbonate (K2CO3) and iodo-methane in aqueous acetone (40°C, 15 h) gave the triol (50) in 80% yield, which was converted to erythronolide A. 

In the transformation of A to B and then to C from the synthesis of 7-deoxypancratistatin, in Section 7.3.A.i., deprotection was followed by conversion of a lactone to a lactam. Provide a mechanism for the deprotection of both the O—MOM group and the dioxolane, giving a triol, and also for the lactone-lactam conversion in the second step. 

The diol 59 was used for an ingenious synthesis of L-mycarose (62, 2,6-dideoxy-3-C-methyl-L-n bo-hexose) and olivomycose (63, 2,6-dideoxy-3-C-methyl-L-ara /no-hexose). The sequence of reactions consisted in 1 epoxidation of the double bond and separation of both stereoisomeric epoxides formed, 2° reduction of the oxirane ring to form two triols 60 and 61, and 3° ozonolytic degradation of the aromatic ring to a carboxyl group. Each trihydroxyacid was converted into a lactone and the lactones were eventually reduced to the required branched-chain sugars 62 and 63. 

The last step of the synthesis of —)-frontalin is presented in Scheme 5.37. Lactone and the ester group in 9 are reduced to Ce triol, where two vicinal OH groups are protected as ketal 10. This permits selective oxidation of the terminal OH group to aldehyde 11, which in the Grignard reaction afforded sec alcohol 12. Oxidation of this alcohol under conditions used for the oxidation of 10 to 11 afforded ketone 13. In the last step, acid-catalyzed transketalization afforded (-yfrontalin TM 5.16 where a (5/ ) stereogenic center is generated under asymmetric induction. 

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