Silylformylations 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-... 

Intramolecular cyclization is particularly effective to terminal alkynes containing three or four methylene units between acetylenic and silyl moieties such as 158, 160, and 162. But-3-ynylmethylphenylsilane does not give any positive result for CO incorporation. Regioselectivity for the silylformylation is completely reversed from the one in the standard silylformylation discussed in Section 11.14.2.2. A bulky /ft7-butyl group in 162 (R = Bu ) plays an... 

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. " ... 

In sharp contrast to simple alkenes that undergo silylcarbonylation, genuine silylformylation of an olefinic moiety is attained only by an intramolecular technique (Equation (33)). Since the most serious competitive reaetion is an intermolecular hydrosilylation reaction to form 192, the key for the successful transformation depends on an appropriate combination of substituents R and R. ... 

Selectivity for the cyclopentenone formation is improved by an intramolecular version. For example, 1,6-hepta-diyne (256, X = CH2) reacts in benzene with 2molarequiv. of Bu Me2SiH to give two bicyclo[3.3.0]octenones, 257 and 258, in 14% and 63% yield, respectively, at 95 °G under GO (20 atm) pressure.When benzene is replaced by acetonitrile, 258 becomes a sole product (60% yield). On the other hand, the reaction at 25 °G gives normal silylformylation product 259 as a major component (59%) with the concomitant formation of 258 (20%) (Scheme 12). ... 

It is worth noting that the reaction of 284 with Me2PhSiH gives 285 as a sole cyclic compound a simple silylformylation product is formed in less than 3% yield (Equation (49)). The outcome is rationalized by intervention of 286 in which intramolecular coordination of an olefinic moiety dominates the orientation of silylrhodation... 

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)). ... 

Two studies of an intramolecular variation of the silylformylation reaction have been reported. In a series of substrates having the acetylene and the hydrosilane as part of the same molecule, silylformylation proceeds intramolecularly with high regio- and stereoselectivity to yield the exocyclic aldehydes in high yields [Eq. (49)].124... 

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

Although the regioselectivity for the silylformylation of 1-alkynes is excellent, that of internal alkynes is low except for 2-alkynoates (vide supra). Also, in the reactions of 1-alkynes, the silyl group is always delivered to the terminal position and the formyl to the C-2 position, thus it is impossible to synthesize 3-silyl-2-alkenals, which requires opposite regioselectivity. Intramolecular directed reactions can circumvent these limitations and expand the scope of the silylformylation of alkynes. 

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... 

Functionalized oxosilacycloheptane 168 was obtained by silylformylation of alkynol 167, in which the catalytic intermediate of an intramolecular hydrosilylation is intercepted by carbon monoxide (Equation 35) <1995JA6797>. 

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-... 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

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. ... 

---