Silylformylation, intramolecular

This tandem intramolecular silylformylation/Sakurai reaction has successfully been applied in a formal total synthesis of mycoticin A [75]. The scope and utility of these reactions was expanded to (Z)- and (E)-crotyl groups leading to the stereospecific incorporation of both anti and syn propionate units into the growing polyol chain (Scheme 21) [76]. 

One of the important advantages of the intramolecular alkene silylformylation reaction as an aldol equivalent is that the products are masked 3,5-dihydroxyalkanals, and therefore that no manipulations are required prior to iteration of the process by aldehyde al-lylation to set up the next intramolecular silylformylation. Given that allylsilanes are well-known aldehyde allylation reagents, intramolecular silylformylation employing a diallylhydrosilane would, in principle, allow for the possibility of a tandem silylformyla-tion/allylsilylation reaction. This has been reduced to practice the diaUylsilyl ethers 60 were subjected to the previously developed silylformylation conditions and the unpuri-fied reaction mixtures were subjected to the Tamao oxidation ]36] to provide triols 61... 

Alkynes are viable substrates for the tandem intramolecular silylformylation/allylsilyla-tion reaction. The 1,5-diastereoselechvity is reversed from that observed with alkene substrates, as the 1,5-anti product predominates (Scheme 5.24) [38]. Synthetic flexibility was demonstrated in that either of two carbon-silicon cleavage procedures may be employed. Protodesilylahon with tetra-n-butylammonium fluoride followed by acetylation (determi-... 

A hydrosilyloxy moiety also works as a directing group for the intramolecular silylformylation regardless of terminal and internal alkynes (Table 9)7 ... 

This type of intramolecular silylformylation is applicable to 4-silyloxy-l,6-diyne 176 which gives 177 in good yields (Equation (31)). The remaining alkynyl moiety in 177 can further undergo intermolecular silylformylation. " ... 

The silyl group in 311, prepared by intramolecular silylformylation, is intermolecularly substituted by a phenyl group by a palladium-catalyzed reaction with Phi to give coupled product 312 (Equation (56)). ... 

A second example of intramolecular silylformylation involves reactions of terminal and internal alkynes linked to a dimethylsiloxy group, as depicted in Eq. (50).125... 

Intramolecular geometries, database studies, 1, 597 Intramolecular hydroalkoxylation, and etherification, 10, 672 Intramolecular pinacol coupling to cyclic 1,2-diols, 11, 51 with samarium reagents, 11, 60 Intramolecular silylformylation, alkynes and alkenes, 11, 489 Intramolecular solvomercuration alkenes, 2, 436 alkynes, 2, 439... 

Tandem intramolecular silylformylation-allylation reaction of diallylhydrosilyl ethers derived from homoallyl alcohols is convenient for rapid, stereoselective synthesis of 1,3,5-triols convertible to more oxygen-functionalized compounds (Scheme 12).142,142a,142b 143 [ he second uncatalyzed allylation step would be facilitated by the formation of a strained silacycle intermediate, which has enough Lewis acidity to activate the formyl group. A similar tandem reaction via alkyne silylformylation has been reported.144... 

Oxasilacyclopentanes have been extensively studied over the period reviewed and many different methodologies have been developed for their syntheses. Intramolecular radical cyclizations <2001JOC1%6>, intramolecular silylformylation of silyloxyalkynes promoted by catalysts such as Rh <20010L1303>, Ni <2001TL3259>, hydrosilylation of homo-propargyl hydrodimethylsilyl ethers promoted by solvated Pt-atoms (Equation 49) <1998JOM57>, and also photoin-duced intramolecular cyclisation of alkynyl-substituted pentamethyldisilanes have been reported <1998TL6891>. 

Although the reaction of 1-alkynes always affords 2-formyl products as shown in Eq. 66, this regiochemistry can be completely reversed using intramolecular silylformylation via exo-dig cyclization of alkynylsilanes and alkynyloxysi-... 

The silylformylation of 1-alkynes has been studied extensively affording (Z)-l-silyl-2-formyl-l-alkenes high regioselectivity. However, internal alkynes have not demonstrated high regioselectivity. Biffis et al. have reported using a cationic dirhodium(II) complex to achieve improved chemoselectivity for the silylformylation of internal alkynes.In addition, intramolecular silylformylation reactions were developed to aid in the regioselectivity of silylformylation of internal alkynes. ... 

Scheme 7.30 Intramolecular silylformylation/cross-coupling promoted by hydrated KF.

Intramolecular silylformylation of alkenes gives five-membered cyclic silyl ethers, the silicon atom of which serve as a Lewis acid to promote crotylation of the resulting formyl group intramolecularly. The remaining C-Si bond is oxidized to give 1,3,5-triols in a diastereoselective manner (Scheme 3-127). 

Bis(allyl)homoallyloxysilanes 56a and 56b are designed for a tandem intramolecular silylformylation-allylsUylation reaction, which has turned out to be an efficient approach to construct polyol and polyketide frameworks [21], For example, heating a solution of 56 in benzene at 60 °C in the presence of Rh(acac)(CO)2 under CO atmosphere followed by the Tamao oxidation gives syn,syn-triols 59 stereoselectively via oxasilacyclopentanes 57 and 58 (Scheme 5.14). Bis(ds-cro-tyl)silane 56b is readily prepared by double Pd-catalyzed 1,4-hydrosilylation of 1,3-butadiene with dichlorosilane followed by reduction with UAIH4 and alcoholysis with the corresponding homoallylic alcohol. 

---