Allylsilanes
Allylsilanes
Allylsilanes
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
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