Esters, reaction with DIBAL

Compound 108 was subjected to the following sequence of reactions Wittig oleflna-tion with (Et0)2P(0)CH2C02Et, reduction of an ester group with DIBAL-H to obtain 111, Simmons-Smith cyclopropanation, and the corresponding xanthate preparation with NaH-CS2-MeI in THE (Scheme 30.19). 

With the fully functionalized heterocyclic core completed, synthetic attention next focused on introduction of the 3,5-dihydroxyheptanoic acid side-chain. This required initial conversion of the ethyl ester of 35 to the corresponding aldehyde through a two-step reduction/oxidation sequence. In that event, a low-temperature DIBAL reduction of 35 provided primary alcohol 36, which was then oxidized to aldehyde 37 with TRAP. Subsequent installation of the carbon backbone of the side-chain was accomplished using a Wittig olefination reaction with stabilized phosphonium ylide 38 resulting in exclusive formation of the desired -olefin 39. The synthesis of phosphonium ylide 38 will be examined in Scheme 12.5 (Konoike and Araki, 1994). 

The Peterson reaction of the chlorovinyl-complex with ethyl trimethylsilylacetate provided the 11Z isomer preferentially (77%), and the 1 IE isomer as a secondary product (15%). The ester was transformed into the C 8 ketone (PhsSnCfy, BuLi, Et20, 79%). Reaction with (/Pr0)2P(0)CH2CN afforded the 1 lZ-retinonitrile in 73% yield. The complex was removed by CuC (72%) and DIBAL-H reduction led quantitatively to 1 lZ-retinal, Fig. (24). 

The iV-aminopyrrole - benzene ring methodology has been applied to a synthesis of the 9,10-dihydrophenanthrene juncusol (218) (81TL1775). Condensation of the tetralone (213) with pyrrolidine and reaction of the enamine with ethyl 3-methoxycarbonylazo-2-butenoate gave pyrrole (214). Diels-Alder reaction of (214) with methyl propiolate produced a 3 1 mixture of (215) and its isomer in 70% yield. Pure (215) was reduced selectively with DIBAL to the alcohol, reoxidized to aldehyde, and then treated with MCPBA to generate formate (216). Saponification to the phenol followed by O-methylation and lithium aluminum hydride reduction of the hindered ester afforded (217), an intermediate which had been converted previously to juncusol (Scheme 46). 

The Homer-Emmons addition of dialkyl carboalkoxymethylenephosphonates to aldehydes [22] has been widely used to generate a,p-unsaturated esters which, in turn, can be reduced to allylic alcohols. Under the original conditions of the Homer-Emmons reaction, the stereochemistry of the oc,(3-unsaturated ester is predominantly trans and therefore the trans allylic alcohol is obtained upon reduction. Still and Gennari have introduced an important modification of the Homer-Emmons reaction, which shifts the stereochemistry of the a,[i-unsaturated ester to predominantly cis [23], Diisobutylaluminum hydride (DIBAL) has frequently been used for reduction of the alkoxycarbonyl to the primary alcohol functionality. The aldehyde needed for reaction with the Homer-Emmons reagent may be derived via Swern oxidation [24] of a primary alcohol. The net result is that one frequently sees the reaction sequence shown in Eq. 6A. 1 used for the net preparation of 3E and 3Z allylic alcohols. 

Barrett used the reaction at the start of his synthesis of an antibiotic.12 The HWE reaction with the enal 47 gives the diene ester 48 and by reduction with DIBAL, the dienol 49. 

The key intermediate in the total synthesis of furaquinocin was obtained in good yield by a reductive Heck reaction that proceeded with a sterically hindered base pentamethylpiperidine (PMP) <02JA11616>. A new hypothesis for the major skeletal rearrangement (anthraquinone —> xanthone —> coumarin) that occurs in the complex biosynthesis of aflatoxin Bi was proposed. To test this hypothesis, an intermediate 11-hydroxy-O-methylstergmatocystin (HOMST) was synthesized as shown below. The key transformation in this synthesis involved the treatment of an ester-aldehyde with Pr3SiOTf, which smoothly produced a mixed acetal. Direct reduction with DIBAL-H led to the aldehyde. The desired product was eventually obtained via several steps as shown <02JA5294>. 

The use of DIBAL-H to reduce nitriles to aldehydes has been added, as has the low-temperature reduction of esters with DIBAL-H to produce aldehydes. Several problems have been added that include these reactions in synthesis. 

Newly added reactions are DIBAL-H reduction of esters, and dialkylcuprate reaction with acid chlorides to produce ketones. 

The ester was cleaved by reduction with DIBAL (Ml AlH) and an achiral version of the normal protecting group put in place. It would obviously be silly to create unnecessary diastereo meric mixtures in these reactions. Then the tin could be exchanged first with lithium and then with an elec-tuophile, even an alkyl halide, with retention of configuration and without loss of enantiomeric purity. The intermediate organolithium compound must have had a stable configuration. 

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