Triflate, trimethylsilyl Diels-Alder reactions

Trimethylsilyl triflate itself can not promote allylation reactions of aldehydes with allyltri-methylsilane. By using the more highly reactive system Me3SiB(OTf)4, the reactions proceed smoothly.326 A very small amount (0.2-1 mol.%) of Me3SiB(OTf)4 is enough for the reactions (Scheme 75). Allylation of acetals can be promoted by trimethylsilyl bis(trifluoromethanesulfo-nyl)imide (Me3SiNTf2),327 which is also a reactive catalyst for Diels Alder reactions.328... 

Selective cyclization of an alkenyl imine is catalyzed by trimethylsilyl triflate (Scheme 76).329 /-Butyldimethylsilyl triflate ( BuN SiOTf) catalyzes imino Diels-Alder reactions of TV-phenyl-aromatic aldimines to afford exo adducts preferentially.330 When A1C13 is used instead of Bufv SiOTf, endo adducts are obtained predominantly. 

Diels-Alder reactions. In the presence of trimethylsilyl triflate, this orthoester is converted into a 1,1-diethoxyallyl cation, CH2=CHC+(OC2H5)2, which reacts with 1,3-dienes at -78° — 0° to give the corresponding adducts of ethyl acrylate. In the presence of trimethylsilyl triflate, ethyl acrylate can undergo Diels-Alder reactions, but higher temperatures are required and yields are lower. 

The synthesis of urdamycinone B (182) by Toshima et al. [76] is an instructive example of the know-how that has been acquired in angucycline chemistry. They directly coupled the unprotected olivose (188) with naphthol 187 using trimethylsilyl triflate as the Lewis acid to afford the C-glycoside 189 after removal of the benzyl ethers and air oxidation in 21 % yield. The strategy essentially developed in our group was then employed in the Diels-Alder reaction of 189... 

Danishefsky and coworkers used a dioxolenium mediated Diels-Alder reaction between 83 and 85, generated from 84, in their total synthesis of dysidiolide (87) (equation 26). The Diels-Alder reaction, using trimethylsilyl triflate as the dioxolenium generating species, proceeded with high facial, endo and regioselectivity affording 86 as the main product, together with 5% of a yet unidentilied stereoisomer. 

Scheme SI presents an entirely different mode of catalyzed [4 2] cycloadditions. Thus trimethylsilyl triflate or triflic acid apparently transform vinyl orthoesters and acrolein acetals to powerful transient allyl cation dienophiles (214) and (219), respectively. The ionic Diels-Alder reactions (213) - (216) and (218) - (220) proceed at low reaction temperatures in a remarkably regio- and em/o-selective man-...

Vinyl triflates can be coupled efficiently to various organotin compounds. Further details of this chemistry have now appeared including efficient preparations of trimethylsilyldienes (Scheme 33).71 Two other routes to silyldienes are outlined in Scheme 34. Reduction of allenic alcohols (21) gave moderate yields of the dienyl products, although the stereoselectivity of the process leaves much to be desired. The second method, involving hydroboration of the allene (22) is much more stereoselective and enables either isomer of the final product to be obtained depending on the elimination conditions used. A key application for such dienes is in Diels-Alder reactions, where the products have vinylsilane functionality for further modification. This type of chemistry has now been explored with 2,3-bis(trimethylsilyl)buta-1,... 

Benzynes generated by the fluoride-induced elimination of TMS and OTf groups from o-trimethylsilyl [6]- and [7]-heUcenol triflates selectively undergo an intramolecular Diels-Alder reaction with one of the benzene rings within the hehcenes, rather than an external furan molecule, giving rise to bridged derivatives of coro-nene (Equation 12.42) [86). 

The benzyne created from 2-(trimethylsilyl)phenyl triflate was used in a highly diastereoselective aryne Diels-Alder reaction with a diene bearing Oppolzer s sultam. This approach to cis-functionalized 1,4-dihydronaphthalenes was reportedly the first aryne Diels-Alder reaction to provide enantioenriched cyclo-adducts. An unusual route to /3-aminoketones involves the treatment of 2-(trimethylsilyl)phenyl triflate with TBAF and an asymmetric vinyldihydropyridone. The resultant aryne Diels-Alder cycloadduct undergoes aromatization/eUmination to create the )V-acyl-/3-aminoketone (eq 13). This method was featured in a multistep synthesis of an unnatural a-amino acid. 

The catalytic enantioselective hetero-Diels-Alder reactions between ketomalonate and activated and non-activated conjugated dienes yield cycloadducts which can be converted into chiral COa-synthons. The intramolecular Diels-Alder reaction of sulfolene masked diene synthon (169) yield 5-carbomethoxy yohimbine systems (170) and (171) with ejco-stereochemistry (Scheme 65). Trimethylsilyl triflate catalyses... 

An extensive review of recent advances in transition-metal-ffee carbon-carbon and carbon-heteroatom cycloadditions of arynes, generated from 2-(trimethylsilyl)aryl triflates, has been presented. The Diels-Alder reaction of arynes, generated from... 

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