Acetals allylsilanes

A kinetic study of reactions of 2,3-dichloro-5,6-dicyano-pflra-benzoquinone (140, DDQ) with silyl enol ethers, silyl ketene acetals, allylsilanes, enamino esters and diazomethanes has been carried out in acetonitrile and DCM, allowing correlations with nucleophilicity parameters for the latter species to be examined. These are found to be 2-5 orders of magnitude larger than expected for Single Electron Transfer processes, supporting a polar mechanism for C-C bond formation at C(5). However, rate constants for (9-attack do correlate well with calculated values assuming rate-determining SET. 

Similarly, trimethylsilyl inflate can be used as a catalyst for the alkylation of 2 methoxy 1,3-oxazolidines [104] or 1-acetoxyadamantane [105] with allylsilane and for the reduction of acetals to ethers with trialkylsilanes [106]... 

Allylsilanes are available by treatment of allyl acetates and allyl carbonates with silyl cuprates17-18, with antarafacial stereochemistry being observed for displacement of tertiary allyl acetates19. This reaction provides a useful asymmetric synthesis of allylsilanes using esters and carbamates derived from optically active secondary alcohols antarafacial stereochemistry is observed for the esters, and suprafacial stereochemistry for the carbamates20,21. 

IA solution of the allylsilane (2 mmol) in chloroform (2 ml) was added all at lonce to boron trifluoride-acetic acid complex (2.2 mmol) with vigorous Ishaking until a single phase resulted. After 5 min, the solution was poured linto saturated sodium hydrogen carbonate solution (10ml) and extracted... 

Allyl acetates can similarly be transformed into allylsilanes by treatment with bis(silyl)cuprates (5). 

Aubele et al. studied the aqueous Prins cyclization using cyclic unsaturated acetals as oxocarbenium ion progenitors and allylsilanes are used as nucleophiles. Cyclizations proceed efficiently inside Lewis acidic micelles (of cerium salt) in water. A variety of vinyl- and aryl-substituted tetrahydropyrans with excellent stereocontrol was obtained (Eq. 3.26).113... 

A one-pot reaction between a tryptophan ester, benzotriazole, and 2,5-dimethoxytetrahydrofuran in acetic acid gives the diastereomeric benzotriazolyl tetracycles, 349, in good yield. Substitution of the benzotriazole by reaction with silyl enol ethers and boron trifluoride etherate gives the corresponding ketones 350 and 351, and reaction with allylsilanes gives the corresponding alkenes 352 and 353. If the boron trifluoride etherate is added to the mixture before the silane, elimination of benzotriazole from 349 is also observed (Scheme 83) <1999T3489>. 

Allylation of aldehydes or ketones using allylsilanes, known as the Hosomi-Sakurai reaction, is a useful method for obtaining homoallylic alcohols. TiIV compounds have been successfully applied to this reaction (Scheme 21) 80 Besides aldehydes and ketones, acylsilanes, 0,0-acetals, and A-,(7-acetals can be employed.81-83 1,4-Addition of an allyl group to an a,/ -unsaturated ketone has been also reported.84... 

In reactions of chiral aldehydes, TiIV compounds work well as both activators and chelation control agents, a- or A-oxygcnated chiral aldehydes react with allylsilanes to afford chiral homoallylic alcohols with high selectivity (Scheme 22).85 These chiral alcohols are useful synthetic units for the synthesis of highly functionalized chiral compounds. Cyclic chiral 0,0- and A/O-acetals react with allylsilanes in the same way.86,87 Allenylsilanes have also been reported as allylation agents. 

These reactions were carried out using /V-mesyIoxazaborolidine 96 (0.5 eq.) and allylsilane 98 (1.5 eq.) in CH2CI2 at —50°C. Conversion of 97 as high as 63% was observed, and the remaining (2S,4R)-97 was recovered in 92% ee. Modification of the electronic nature of the aryl substituent attached to the acetal carbon at the para position of 97 did not affect the enantioselectivity of ring cleavage. 

Various carbon nucleophiles, such as allylsilanes, allylstannanes, silyl enol ethers, ketene silyl acetals, organoaluminum compounds, and Grignard reagents were effective as carbon nucleophiles. 

The alkoxycarbenium ions generated by the cation pool method react with various carbon nucleophiles such as substituted allylsilanes and enol silyl ethers to give the corresponding coupling products in good yields. It should be noted that the reactions of alkoxycarbenium ion pools with such nucleophiles are much faster than the Lewis acid promoted reactions of acetals with similar nucleophiles. A higher concentration of the cationic species in the cation pool method seems to be responsible. 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

The undefined mechanism of the aldol-type Mukaiyama and Sakurai allylation reactions arose the discussion and interest in mechanistic studies [143-145]. The proposed mechanism was proved to proceed through the catalytic activation of the aldehyde and its interaction with the silyl ketene acetal or allylsilane producing the intermediate. From that point the investigation is complicated with two possible pathways that lead either to the release of TMS triflate salt and its electrophihc attack on the trityl group in the intermediate or to the intramolecular transfer of the TMS group to the aldolate position resulting in the evolution of the trityl catalyst and the formation of the product (Scheme 51). On this divergence, series of experimental and spectroscopic studies were conducted. 

Maeda K, Shinokubo H, Oshima K (1997) Lewis acid-induced chemo- and stereoselective allylation of a-iodo mixed acetal with allylsilane. J Org Chem 62 6429-6431... 

The first total synthesis of the marine dolabellane diterpene (- -)-4,5-deoxyneodolabelline 106 has been accomplished by Williams et alf" The /ra j -disubstituted dihydropyran moiety in key intermediate 69 was efficiently prepared from mixed acetal 103 by RCM with catalyst C and subsequent Lewis acid-catalyzed allylation of ethyl glycosides 104 with allylsilane 107 (Scheme 18). 

Homoallylk ethers. lodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

An analogous process, which provides a nucleophilic allylsilane, has been reported, employing (Me3Si)3Al etherate in conjunction with allyl acetates and Pd° (equation 113).225... 

Significantly better results in addition of non-stabilized nucleophiles have come from hydrogenolysis reactions using formate as a hydride donor as shown in Scheme 8E.46. The racemic cyclic acetate and prochiral linear carbonates were reduced in good enantioselectivities by monophosphine ligands (/ )-MOP (16) and (Zf)-MOP-phen (17), respectively [195]. The chirality of the allylsilane can be efficiently transferred to the carbinol center of the homoallylic alcohol by the subsequent Lewis acid catalyzed carbonyl addition reaction 1196], The analogous... 

---