Silyl intramolecular

Besides, the types of nitrile used in the abovementioned reaction have also strong impact on the reaction patterns. When 1,2,3,4-tetra-substituted di-lithio reagents were treated with MesSiCN, a tandem silylation-intramolecular substitution process readily occurred to yield siloles 45, while the reaction of 2,3-di-substituted di-lithio reagents with MesSiCN gave rise to the first synthesis of (Z, Z)-dienylsilanes 46 with high stereoselectivity (Scheme 23) [63]. In this reaction, the normal reaction pattern that nitriles are attacked at C=N triple bond by mono-Li reagents was not found. Instead, cleavage of Si-C bond in Si-CN moiety was observed. 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

In the prostaglandin synthesis shown, silyl enol ether 216, after transmetaJ-lation with Pd(II), undergoes tandem intramolecular and intermolecular alkene insertions to yield 217[205], It should be noted that a different mechanism (palladation of the alkene, rather than palladium enolate formation) has been proposed for this reaction, because the corresponding alkyl enol ethers, instead of the silyl ethers, undergo a similar cyclization[20I],... 

Various bicyclic and polycyclic compounds are produced by intramolecular reactions] 127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring Junctions are selectively generated. In the formation of 158, allyhc silyl ether remains intact. A bridged bicyclo[3.3. l]nonane ring 159 was constructed... 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Pd(II)-catalyzed cyclization of the siloxyhexatriene 34 offers a cyclohexe-none annulation method. The Pd enolate 35, formed by transraetallation of the silyl enol ether with Pd(II), is an intermediate which undergoes intramolecular eWo-alkene insertion. Then Pd(II) is regenerated to give 36, and finally cyclohexenone is formed[38]. 

The Reforrnatsku reaction of a-halogenated carboxylic esters with silylated cyanohydrins combined with an intramolecular acylation reaction gives fluorinated derivatives of tetronic acid [28] (equation 17) It is noteworthy to mention that this particular reaction sequence only proceeds with ultrasonic irradiation A very... 

In this series of amides, hydrolysis or aminolysis of a simple ester, cleavage of a silyl groups a cis/trans isomerization, or reduction of a quinone to a hydro-quinone exposes an alcohol that then induces deprotection by intramolecular addition to the amide carbonyl. 

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Intramolecular Silyl Nitronate-Olefin Cycloaddition (ISOC)... 

Although nitrile oxide cycloadditions have been extensively investigated, cycloadditions of silyl nitronates, synthetic equivalent of nitrile oxides in their reactions with olefins, have not received similar attention. Since we found that the initial cycloadducts, hl-silyloxyisoxazolidines, are formed with high degree of stereoselectivity and can be easily transformed into isoxazolines upon treatment with acid or TBAF, intramolecular silylnitronate-olefin cycloadditions (ISOC) have emerged as a superior alternative to their corresponding INOC reactions [43]. Furthermore, adaptability of ISOC reactions to one-pot tandem sequences involving 1,4-addition and ISOC as the key steps has recently been demonstrated [44]. 

One-pot tandem sequences involving 1,4-addition and ISOC as the key steps have been developed for the construction of N and 0 heterocycles as well as of carbocycles [44]. In this sequence, the nitronate arising from 1,4-addition to an a, -unsaturated nitro alkene is trapped kinetically using trimethyl silyl chloride (TMSCl). The resulting silyl nitronate underwent a facile intramolecular 1,3-dipolar cycloaddition with the unsaturated tether (e.g.. Schemes 20-22). 

Davis has described an approach to related 1,3-diol synthons [49] (Eq. 20). Silylation of the -hydroxy ester 129 with diisopropylchlorosilane, followed by fluoride ion-catalyzed intramolecular hydrosilylation generated a 1 1 diastere-omeric mixture of acetals 131. These acetals were shown to undergo diastereo-selective nucleophilic additions vide infra). 

Difunctional reagents, for example the very cheap dimethyldichlorosilane 48, which is produced on a large technical scale, and the much more reactive and expensive dimethylsilyl bis(O-triflate) 49 [65-67] (Scheme 2.8) convert alcohols or phenols 11 in the presence of bases, for example triethylamine or DBU, into the silylated compounds 50. Thus 48 and 49 and other bifunctional reagents such as di-tert-butyldichlorosilane [68] or di(tert-butylsilyl)-bis(0-triflate) [69] and the subsequently described 51 and 52 combine two alcohols to silicon-tethered molecules 50, which can undergo interesting intramolecular reactions [70-74]. 

CoCl2 6H20 [149] or TMSOTf 20 [150-152] in acetonitrile afford Diels-Alder adducts in yields of up to 85%. The dienals 607 are converted by bis(trimethyl-silyl)thiane 601 or bis(trimethylsilyl)selenide 604 and catalytic amounts of BuLi, via 609, to the intramolecular Diels-Alder products 610 in up to 70% yield [153, 154]. 

Intramolecular asymmetric hydrosilylation-oxidation of (alkenyloxy) hydrosilanes provides an efficient method for the preparation of optically active polyols from al-lylic alcohols. Cyclization of silyl ethers 54 of a meso-type allyUc alcohol in the pres-... 

---