gave the desired product in the absence of TMSOTf (entries 6
and 7). Although use of (E)-2-hexenal as the second electrophile
resulted in the formation of a complex mixture of undesired
products, the corresponding acetal (4f) afforded the tricyclic
product 2af having alkenyl group as R2 (entry 8). We also
examined the variation of the first electrophile. Reaction using
3b in combination with 3a gave 2ba with high selectivity (entry
9). Although we found that use of more sterically demanding 3c
as the first electrophile resulted in much lower yields, only 2ca
without any other tricyclic products were found in the reaction
mixture (entry 10). Presumably, 3c as well as 1 may be
consumed completely in the first step by any side reaction,
resulting in the highly selective formation of 2ca.
To evaluate the role of the boryl group in the cyclization, we
examined the reactions of the related allylsilanes bearing silyl
(5) and methyl (6) groups at the b-positions.§ Under the same
reaction conditions as for the preparation of 2aa from the b-
Scheme 2 Reagents and conditions: a) Li, liq. NH3, t-BuOH, 233 °C; b)
Me3NO•2H2O, diglyme, 160 °C; c) H2O2, NaOH aq., THF, 50 °C.
In summary, we report a cascade cyclization giving trans-
1,2-benzodecaline skeletons via sequential reaction of a-
phenethyl-b-borylallylsilane 1 with aldehydes. The stereo-
chemical aspects and high structural diversity may deserve
further investigation of the present stereoselective cyclization.
Furthermore, the boryl group incorporation at the bridgehead
tertiary carbon atom as a hydroxy equivalent may open up new
possibilities for the synthesis of polycyclic bridgehead alcohols
via cationic cyclization.
boryl counterpart 1, 3 molar equiv. of 3a were reacted with 5
and 6 in the presence of TMSOTf [eqn. (2)]. While the former
(5) gave only a complex mixture of products, the methyl
derivative afforded tricyclic product 8 along with its diaster-
eomer in a ratio of 3+1 in a total yield of 42%. This result
suggests that the sequential reaction with aldehydes giving the
tricyclic skeleton is efficiently controlled by the boryl group
with respect to yield and stereoselectivity. Presumably, the
electronic nature of the b-substituent has the predominant effect
on the formation and cyclization of the cationic intermediates,
e.g., A in Scheme 1.
Notes and references
‡ A general procedure (A) for the three component cascade reaction of 1
with electrophiles. To a mixture of 1 (50 mg, 0.12 mmol) and 3 (0.12 mmol)
in CH2Cl2 (0.12 mL) was added a CH2Cl2 solution of TiCl4 (2.0 M, 74 3
1023 mL, 0.15 mmol) at 278 °C, and the mixture was stirred at 278 °C for
2 h. To this was added 3 or 4 (0.25 mmol) at 278 °C, and the mixture was
stirred at 0 °C for 3 h. Aqueous NaHCO3 (sat.) was added to the mixture.
Extraction with AcOEt followed by silica gel column chromatography
afforded 2. For procedure B, the addition of the second electrophile (3 or 4)
was followed by the addition of TMSOTf (45 3 1023 mL, 0.25 mmol) at
278 °C.
A related cascade cyclization using 1,1,3,3-tetramethoxy-
propane with 1 in the presence of TiCl4 afforded boryl-
substituted trans-1,2-benzodecaline derivative 9 in good yield
§ The requisite b-silylallylsilane 5 and b-methylallylsilane 6 were prepared
by palladium-catalyzed bis-silylation of 5-phenylpenta-1,2-diene5 and
Suzuki-Miyaura cross-coupling of 1 with iodomethane,6 respectively.
1 For reviews on domino, cascade, and tandem reactions, see: L. F. Tietze,
Chem. Rev., 1996, 96, 115; S. E. Denmark and A. Thorarensen, Chem.
Rev., 1996, 96, 137; J. D. Winker, Chem. Rev., 1996, 96, 167; I. Ryu, N.
Sonoda and D. P. Curran, Chem. Rev., 1996, 96, 177; P. J. Parsons, C. S.
Penkett and A. J. Shell, Chem. Rev., 1996, 96, 195; K. K. Wang, Chem.
Rev., 1996, 96, 207; A. Padwa and M. D. Weingarten, Chem. Rev., 1996,
96, 271.
[eqn. (3)]. Interestingly, the relative stereochemistry of the two
methoxy groups was trans, indicating the second carbon–
oxygen bond activation leading to cyclization may involve
chelation of the two methoxy groups onto the titanium metal.
The tricyclic organoboron compounds served as useful
synthetic precursors for the corresponding tertiary alcohols
bearing the hydroxy groups at the bridgehead carbon atoms
(Scheme 2). Thus, treatment of 2aa with trimethylamine oxide
at 160 °C gave the bridgehead alcohol 10 in 85% yield. Further
synthetic elaboration was demonstrated by sequential treatment
of 2aa with Li–NH3 and H2O2, which gave the dienyl alcohol 12
via isolation of 11. An attempt at an alternative pathway to 12
via Birch reduction of 10 resulted in the reduction of the tertiary,
benzylic hydroxy group to give 13 in high yield as a 1+1 mixture
of diastereomers, indicating that the boryl group served as a
masked hydroxy group in the transformation into 11.
2 M. Suginome, Y. Ohmori and Y. Ito, J. Am. Chem. Soc., 2001, 123,
4601.
3 For the synthesis of b-borylallylsilanes by palladium-catalyzed silabora-
tion of allenes, see: M. Suginome, Y. Ohmori and Y. Ito, Synlett, 1999,
1567; S.-y. Onozawa, Y. Hatanaka and M. Tanaka, Chem. Commun.,
1999, 1863; M. Suginome, Y. Ohmori and Y. Ito, J. Organomet. Chem.,
2000, 611, 403.
4 For the related cyclizations of allylsilanes with aldehydes giving
4-halotertahydropyrans, see: L. Coppi, A. Ricci and M. Taddei, J. Org.
Chem., 1988, 53, 913; Z. Y. Wei, D. Wang, J. S. Li and T. H. Chan, J.
Org. Chem., 1989, 54, 5768.
5 H. Watanabe, M. Saito, N. Sutou, K. Kishimoto, J. Inose and Y. Nagai,
J. Organomet. Chem., 1982, 225, 343.
6 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
Chem. Commun., 2001, 1090–1091
1091