Ph. D. THESIS 2009

All the attempts to obtaine the desired bicyclo spiro- bis(1',5'-dioxane) failed
due to the strained geometry of the starting diketone 2, the mass spectrometry
analysis indicating only the presence of spiro-mono(1',5'-dioxane) compounds.
As depicted in Scheme 5, the acetalization reaction was carried out using two
different routes: the classic one (method a), consisting of the azeotropic
distillation, at reflux in benzene, catalyzed by PTSA [24] and under microwave
irradiation (method b), deposited on a solid support of Montmorillonite clay
K10 [25], followed by extraction with solvent in microwave oven.
O O +
OH
OH
R1 a. pTSA, C6H6, reflux
O
b. K10, microwave irrad.
R2 O
O
R1 R2 2 7 R1, R2: CH3 8 R1, R2: CH2Br 3 R1, R2: CH3 4 R1, R2: CH2Br Scheme 5. Acetalization reaction of compound 2 with 2.2-disubstituted-propane-1,3-diols
Table 8. Yields and reaction time for compounds 3 and 4 using method (a) and method (b)
Compound Yield (%) Reaction time Yield (%)
Reaction
Meth. (a)
(day) Meth. (b) time (min)
3 25 8 60 3
4 30 6 65 3
The advantages of the synthesis using method (b) over the method (a) lie in a
remarkably reduced reaction time, less toxic and inexpensive conditions of
reaction, as well as in the significantly increased yields. Also, it is worth
mentioning the absence of harmful solvents such as benzene or toluene, the
PTSA catalyst, and the facile work-up procedure.
1.6.2. NMR spectra in solution
The 1 H NMR spectra (Figure 14 a, b) of spiro compounds 3 and 4 revealed a
semi-flexible structure consisting of an anancomeric carbobicycle and a flexible
1',5'-dioxanic ring.
They display unique signals, with mean values of the chemical shifts, for the
axial and equatorial protons of the heterocycle (positions 2' and 4').
Thus, the spectrum of compound 3 (Figure 14a) exhibits two singlets
(δ=3.31 and δ=3.46 ppm) for axial and equatorial protons at 2'-C and 4'-C as a
consequence of the identical magnetic environment of the two protons and of
the heterocycle flipping. The 6 H singlet at δ=0.91 ppm is ascribed to the
protons of the equivalent methyl groups located at the position 3' of the
dioxanic ring, which exhibit a unique signal due to the flexibility of the ring.
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