In situ cycliza- tion delivered the desired 1,3-diaxially encumbered spiroketal 39 as a single diastereomer in near-quantitative yield eq The carbon—silicon bond is far more resistant to oxidation and hydrolysis than the carbon—boron bond and there- fore can be carried through many steps of a synthetic sequence.
Avoid Skin Contact with All Reagents Cyclization of alcohol 47 affords 49 as a pair of secondary alcohol diastereomers in near-equal amounts,20 while the TMS derivative 48 generates alcohol 50 essentially as a single isomer eq In sharp contrast, similar reduction of bulky vinylsilane 52 affords anti- diol 54, which was protiodesilylated to anti-1,3-diol 55 for direct comparison with syn-diol 53 eq While the above examples show the use of the TMS group to maximal advantage, the requirement of a proximal alcohol for eventual protiodesilylitic removal or oxidation has prompted H H OH In addition to introduction of the valuable UV detection feature, the PDMS moiety is a routinely employed surrogate for the hy- droxyl group, as seen in the conversion of 59 to lactone 60 by application of the KFT oxidation eq Landais has shown that intramolecular oxyselenylation of alco- hol 61 undergoes cyclization exclusively via conformation 61a to deliver tetrahydrofuran 62 to the exclusion of isomer 63 eq Min- imally protected dibenzyl pentaol 73 undergoes acid-mediated Peterson elimination27 to 74, base-mediated Peterson reaction to 76, and KFT oxidation to inositol derivative 75 eq The argument is made that the steric demands of the OTIPS group provide a higher concentration of the requisite s-cis conformation of 77 eq In this study, Helal, Magriotis, and Corey conclusively demonstrated that the distal acetylenic TIPS moiety was responsi- ble for catalyzed delivery of the activated hydride essentially only from the indicated ketone lone pair eq Addition of metalated dithiane to epoxide affords intermediate that upon warming in the presence of epoxide and HMPA undergoes Brook rearrangement to with concomitant addition to epoxide , leading to spongistatin segment Lutgens, F.
K; Tarbuck, E. Essentials of Geology, 7th ed. Synthesis , Korean Chem. Gauthier, D. Bracegirdle, S.
Nishiyama, H. Heitzman, C. Hayakawa, I. Iwamoto, M. Shea, K. For a review, see: b Takeda, T.
Heteratom Chem. Aeilts, S.go to link
Marjanovic, J. Urabe, H. Wilson, S. Fleming, I. James, P. Peterson reaction: a Ager, D.
Heo, J. B; Roush, W. Rucker, C. Yamamoto, Y.
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Miller, R. Brook, A. Smith, A. Philip L. Preparative Methods: commercially available but expensive. Several preparations of 1 2—4 or the precursor alcohol 2 5,6 have been reported. The simplest involves the direct silylation of methallyl alcohol eq 1. TMSCl 3. Original Commentary Daniel A. The development by Trost and co-workers of palladium—trimethylenemethane TMM cycloadditions employ- ing the title reagent 1 and related reagents was a seminal advance in ring-construction methodology.
Many lines of evidence indicate that in the presence of catalytic Pd0, 1 forms the reactive intermediate palladium—TMM complex 3. The ion- pair nature of the intermediate 4 is thought to be the source of some unusual stereochemical effects which give the reactions some of the characteristics of concerted cycloadditions. The use of optically pure vinyl sulfoxides al- lows for reasonable asymmetric induction eq 9.
AlthoughPd—TMMcycloadditions with alkynes are unsuccessful, the formation of methylene- cyclopentenes from alkynes can be accomplished via the cyclo- pentadiene adduct eq Aldehydes can be converted to methylenete- trahydrofurans using tributyltin acetate or trimethyltin acetate as a cocatalyst eq The cocatalyst tris acetylacetonato indium can be used to favor the 1,2-cycloaddition. These reactions have also been accomplished using nickel catalysts.
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As discussed above, one of the sim- plest methods of preparation of 3-acetoxytrimethylsilylmethyl- 1-propene 1 is the direct silylation of the C,O-dianion derived from methallyl alcohol, followed by O-desilylation and subse- quentacetylation eq1. Alternatives to 1 include the carbonate derivatives 7. More recently, Organ and Murray have proposed a conve- nient, one pot synthesis of 1, starting from 2-chloropropenol eq Additionally, the use of 1 as an nucleophilic allylating agent is covered.
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Pd-TMM Cycloadditions. The challenge was to over- come the poor nucleophilicity of the intermediate alkoxide, result- ing from the Pd-TMM nucleophilic addition to the aldehyde. The use of a more electropositive and oxyphilic cocatalyst such as In acac 3 favors 1,2-additions over 1,4-additions eq So far, all the known examples involve tropone derivatives as starting mate- rials.
A recent example includes the addition of a methyl-substituted analog of 1 to 2-phenyltropone. Allylation of Aldehydes. Nakata and co-workers have used 1 for the C-glycosidation of 20 on the way to the total synthesis of Brevetoxin-B eq Allylsilane 1 has been used in the Ferrier C-glycosylation of tri-O-acetyl glucal 23 eq 46 and acetamidate 24 eq The rich chemistry of the resulting allyl acetates e. Alternatively, 27 was re- acted with N-chlorosuccimide and benzylamine to generate the corresponding sulfonyl chloride that was reacted in situ with benzylamine to give a separable mixture of diastereomeric sulfonamides Both sulfonamides underwent intramolecular Pd-catalyzed allylation giving thiazonine derivatives 31 eq How- ever, better yields are obtained with the methanesulfonate analog of 1.
No formation of the corresponding trimethylenemethane-ruthenium complex was observed eq Related reagents include carbonate43,67 and sulfonate63—65 derivatives of alcohol 2, other halide deriva- tives of 6, 60,68—70 and corresponding trialkyltin22,71 derivatives. Trost, B. Agnel, G. Knapp, S. Cossy, J. Ishibashi, H. Chaigne, F. Jones, M. Bambal, R.
Organ, M. Sakar, T. In Science of Synthesis; Fleming, I. Singleton, D.
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