Allylsilanes
Allylsilanes
Allylsilanes
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Scheme 97 Synthesis of Homoallylic Ethers [79,237]<br />
Cl<br />
SiMe 3<br />
28 R 1 = H; X = Cl<br />
+<br />
192 R 1 = Bn<br />
Ph OMe<br />
SiMe 2<br />
OMe<br />
Bn<br />
+<br />
BF 3 OEt 2<br />
85%<br />
OEt<br />
OEt<br />
Ph<br />
Cl<br />
OMe<br />
247 major isomer<br />
TiCl 4, CH 2Cl 2<br />
−78 o C, 2 h<br />
Intramolecular cyclization of allylsilanes with an acetal function can provide a variety of<br />
heterocyclic compounds. For example, tetrahydropyran 250 is prepared in 92.8% ee from<br />
hydroxy-substituted chiral allylsilane (S,E)-249 (93.2% ee) (Scheme 98). [289] Here, silane<br />
(S,E)-249 was prepared by the intramolecular silylsilylation protocol described in Section<br />
4.4.40.20. In this case, intramolecular cyclization proceeds with nearly complete chirality<br />
transfer.<br />
Scheme 98 Asymmetric Synthesis of Tetrahydropyrans [289]<br />
HO<br />
( ) 3<br />
SiMe2Ph<br />
Bu<br />
(S,E)-249 93.2% ee<br />
iPrCHO<br />
TMSOTf<br />
−78 oC 99%<br />
O<br />
Pr i<br />
OTMS<br />
SiMe 2Ph<br />
Bu<br />
Bn<br />
248<br />
OEt<br />
O Pr i<br />
Bu<br />
250 92.8% ee<br />
2-Benzyl-1-methylbut-3-enyl Ethyl Ether (248): [237,311]<br />
To polystyrene-supported allylsilane 192 (500 mg) in CH 2Cl 2 (20 mL) was added 1,1diethoxyethane<br />
(153 mg, 1.3 mmol) and TiCl 4 (250 mg, 1.3 mmol) at ±788C. After 2 h a<br />
sat. aq NaHCO 3 was added. The organic phase was separated and the NaHCO 3-phase was<br />
extracted (3 ”) with CH 2Cl 2. Flash chromatography(MTBE-light petroleum ether [bp 40±<br />
608C] 1:6 to 1:3) of the residue gave the product; yield: 51 mg (0.50 mmol ·g ±1 of 192).<br />
4.4.40.61 Method 61:<br />
Acylation with Acid Chloride<br />
Lewis acid mediated reactions of allylsilanes with acid chlorides provide â,ã-unsaturated<br />
ketones, for example, the synthesis of artemesia ketone (251) (Scheme 99). [52] This synthetic<br />
application took advantage of the preparation method of allylsilane 10 described<br />
in Section 4.4.40.1.<br />
Scheme 99 Synthesis of Artemesia Ketone [52]<br />
O<br />
Cl<br />
+<br />
FOR PERSONAL USE ONLY<br />
4.4.40 <strong>Allylsilanes</strong> 909<br />
10<br />
SiMe 3<br />
AlCl3, CH2Cl2, −60 oC 90%<br />
3,3,6-Trimethylhepta-1,5-dien-4-one (Artemesia Ketone, 251): [52]<br />
A mixture of 3-methylbut-2-enoyl chloride (11.85 g, 0.1 mol) and AlCl 3 (13.35 g, 0.1 mol) in<br />
CH 2Cl 2 (50 mL) was added dropwise over 45 min to a stirring soln of but-2-enylsilane 10<br />
(15.65 g, 0.11 mol) in CH 2Cl 2 (100 mL), previouslycooled to ±608C. The mixture was kept<br />
at this temperature for 10 min after the final addition, and then poured slowlyonto a mix-<br />
Sarkar, T. K., SOS, (2002) 4, 837. 2002 Georg Thieme Verlag KG<br />
O<br />
251<br />
for references see p 920
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