A. B. C. Simas et al. / Tetrahedron Letters 43 (2002) 6893–6895
6895
BnO
O
BnO
O
O
BnO
OH
O
ii
i
7a
O
PhO2S
12, quant.
11, 70%
O
M
13
M = Molybdenum complex
Scheme 6. Reagents and conditions: (i) a. LHMDS, THF, −78°C, b. MoOPH, THF, −78°C. (ii) DBU, THF, rt.
Use of Vedejs’ reagent12 produced a,b-unsaturated
ketone 11 (Scheme 6) instead of the desired cyclic
compound 12. After treatment with DBU (but not with
either pyridine or Et3N), enone 11 was converted to 12.
Interestingly, it appears that, during the first reaction,
formation of less polar product 11 occurs in the reac-
tion medium (TLC) before the usual reductive work-up.
Thus, apparently, the metal peroxide13 intermediate 13
disproportionates spontaneously in situ producing 12,
which suffers retro-Michael-type ring opening under
the basic conditions. Ketone 12 represents an isofla-
vanone model structure.
References
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It is noteworthy that this chemistry enables the prepa-
ration of 3-substituted dihydrobenzopyran systems
employing phenols bearing free ortho positions as start-
ing materials. It renders unnecessary the employment of
2-functionalized phenols, a common feature of the
alternative synthetic methodologies.14
8. These were the highest values for signal effects on vicinal
hydrogen atoms of the dihydropyran ring system.
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13. In their original work dealing with oxidation of enolates,
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13.
In summary, a practical synthesis of chromene-based
vinyl sulphones has been developed. These novel struc-
tures have proven to be quite versatile. Phenyl adducts
resulting from these structures may have their sulphone
group manipulated to yield isoflavan, isoflavene and
isoflavanone models.
Acknowledgements
We thank CNPq (PIBIC-fellowship to LFOF), Central
Anal´ıtica-NPPN and Centro Nacional de RMN-UFRJ
for analytical data, Instituto de Qu´ımica-UFRJ for IR
spectra.
14. Dean, F. M. In The Total Synthesis of Natural Products;
ApSimon, J., Ed.; Wiley-Interscience: New York, 1973;
Vol. 1, p. 467.