Aldolates, boron, oxidation

Condensation catalyst. Boric acid (also boron oxide and 10-hydroxy-10.9-boroxarophenanthrene)123 has been used as catalyst for aldol condensation and subsequent dehydration. The reaction is carried out in refluxing m-xylene under a Dean-Stark trap for removal of water. Examples126 ... 

A number of modifications were made to meet scale-up requirements. In the preparation of the common intermediate, LiBH4 was used in place of LiAlH4 in Step A-2 and a TEMPO-NaOCl oxidation was used in place of Swern oxidation in Step A-3. Some reactions presented difficulty in the scale-up. For example, the boron enolate aldolization in Step B-l gave about 50% yield on the 20- to 25-kg scale as opposed to greater than 75% on a 50-g scale. The amide formation in Step B-3 was modified to eliminate the use of trimethylaluminum, and the common intermediate 17 could be prepared on a 30-kg scale using this modified sequence. The synthesis of the C(l)-C(6) segment V was done by Steps C-l to C-5 in 66% yield on the scale of several kg. 

Our persistence with the boron-mediated aldol reaction of 4 and 5 was rewarded when the reaction was conducted without recourse to the usual oxidative workup. Other work conducted by our group had shown that oxidative cleavage of certain aldol borinates under standard conditions (H2O2, pH 7 buffer, H20/Me0H) led to poor yields of the aldol products. In the case of 44, the oxidative step was omitted and the reaction mixture was placed directly on silica gel and then eluted to afford aldol adduct 44 in excellent yield (89%) and with improved diastereoselectivity (90 10 ds) relative to the corresponding lithium conditions.17... 

Boron-mediated ketone-ketone aldol reactions have been described, using boron enolates formed with dicyclohexylboron chloride and triethylamine.124 Following addition of the acceptor ketone to form a boron aldolate, oxidation with peroxide yields the aldol product. 

Paquette et al. start with the bis-vinylogation of the same compound 29 [14], by Wittig-Horner reaction, reduction, and oxidation (Scheme 5). For the formation of the C17-C16 bond, the onti-aldol 41 (ds not reported) is obtained by treatment of the aldehyde 39 with the (Z)-boron enolate 40, bearing a dithioketal moiety that is later to be the C51-C54 side chain. 3-Hydroxy-assisted, diastereoselective reduction of the keto group at C15 gives 41, which is converted into intermediate 42 in five more steps. The dethioketalization of 41 is achieved with phenyliodine(m) bis(trifluoroacetate) [16], As in Nicolaou s synthesis, the N12-C13 amide bond is formed first, followed by a low-yielding (21%, even at a concentration of 1 him) macrolactonization to 3. Table 1 summarizes the benchmark data of the two total syntheses of sanglifehrin A (1). 

If one now considers what would probably be needed in the synthetic direction once aldehyde 5 had been reached, a -selective Evans asymmetric aldol reaction11 with boron enolate 6 could potentially set the C(21) and C(22) stereocentres in 3. All that would be required subsequently would be product liberation from the auxiliary by reduction, and oxidation at C(20). With compound 3 in hand the stage would then be set for implementation of the second Julia olefination tactic. 

The group-selective aldolization/desymmetrization of w -dialdehyde with a boron enolate of TV-propionylsultam yielded lactols with simultaneous generation of four stereogenic centers. Oxidation followed by saponification of the sultam moiety provided the Prelog-Djerassi lactonic acid in 61-71% overall yields (Equation (183)).689... 

Barton oxidation was the key to form the 1,2-diketone 341 in surprisingly high yield, in order to close the five-membered ring (Scheme 38). The conditions chosen for the deprotection of the aldehyde, mercuric oxide and boron trifluoride etherate, at room temperature, immediately led to aldol 342. After protection of the newly formed secondary alcohol as a benzoate, the diketone was fragmented quantitatively with excess sodium hypochlorite. Cyclization of the generated diacid 343 to the desired dilactone 344 proved very difficult. After a variety of methods failed, the use of lead tetraacetate (203), precedented by work performed within the stmcmre determination of picrotoxinin (1), was spectacularly successful (204). In 99% yield, the simultaneous formation of both lactones was achieved. EIcb reaction with an excess of tertiary amine removed the benzoate of 344 and the double bond formed was epoxidized with peracid affording p-oxirane 104 stereoselectively. Treatment of... 

Stereoselective aldol condensations. Ketones react with 1 in the presence of ethyldiisopropylamine to form (Z)-boron enolates with high stereoselectivity. These derivatives react with aldehydes to afford primarily erythro- -ketols after oxidation and alkaline hydrolysis. ... 

Monohydroboration of 1-alkynes followed by oxidation gives the corresponding aldehydes in high yields.-" Oxidation of the vinyl carbon-boron bond produces the enol, which then tautomerizes to the carbonyl group. To minimize aldol condensation of the aldehyde formed during oxidation, the reaction should be carried out at pH 8 or in buffered medium. " ... 

Given this problem, the attachment of the butanone synthon to aldehyde 74 prior to the methyl ketone aldol reaction was then addressed. To ovenide the unexpected. vTface preference of aldehyde 74, a chiral reagent was required and an asymmetric. syn crotylboration followed by Wacker oxidation proved effective for generating methyl ketone 87. Based on the previous results, it was considered unlikely that a boron enolate would now add selectively to aldehyde 73. However, a Mukaiyama aldol reaction should favour the desired isomer based on induction from the aldehyde partner. In practice, reaction of the silyl enol ether derived from 87 with aldehyde 73, in the presence of BF3-OEt2, afforded the required Felkin adduct 88 with >97%ds (Scheme 9-29). This provides an excellent example of a stereoselective Mukaiyama aldol reaction uniting a complex ketone and aldehyde, and this key step then enabled the successful first synthesis of swinholide A. 

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