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Helvetica Chimica Acta – Vol. 92 (2009)
decanted from a very sticky red-brown residue through a glass frit. To ensure complete removal of 17, the
residue was redissolved in 20 ml of CH2Cl2; 125 ml of hexane was added, causing further precipitation of
O¼PPh3. This mixture was decanted through a glass frit. This procedure was repeated twice. The filtrate
was concentrated by rotary evaporation to a white-yellow solid, which was washed repeatedly with
hexane and filtered to extract 17, which was obtained as a yellow liquid upon rotary evaporation. A yield
was not calculated, for 17 was used crude in the next reaction due to its presumed instability. 1H-NMR
1
2
(CDCl3): 6.54 (dd, J(13C,H) ¼ 168, J ¼ 10, 1 H); 4.76 (dd, J(13C,H) ¼ 5.7, J ¼ 10, 1 H); 2.90 (m, 4 H);
2.05 – 2.16 (m, 1 H); 1.88 – 2.02 (m, 1 H). 13C-NMR (CDCl3): 134.2 (Br2C¼CH).
1,1-Dibromo-3,3-diethoxy[2-13C]prop-1-ene (18c). Conversion of 17 to 18c employed the dethio-
acetalization procedure of Stork and Zhao [53]. To a soln. of crude 17 (from 3.21 g (21.5 mmol) of 16) in
40 ml of dry EtOH was added all at once [bis(trifluoroacetoxy)iodo]benzene (14.0 g, 32.6 mmol;
Aldrich). The soln. was stirred 40 min at 208, then it was poured into 70 ml of sat. aq. NaHCO3 soln. This
mixture was extracted with Et2O (4 ꢁ 60 ml). The combined org. layers were washed with 50 ml of H2O
and dried (MgSO4). Rotary evaporation, followed by flash CC through SiO2, yielded 18c as a clear,
slightly yellow liquid. The overall yield for transformation of 16 to 18c was 1.24 g (4.30 mmol, 20%). IR
(film): 3026w, 2978m, 2930w, 2880w, 1685w, 1594w, 1475w, 1444w, 1370m, 1333m, 1117s, 1058s, 763m.
1H-NMR (CDCl3): 6.57 (dd, 1J(13C,H) ¼ 166, J ¼ 6.5, 1 H); 5.07 (d, J ¼ 6.5, 1 H); 3.52 – 3.75 (m, 4 H); 1.24
(t, J ¼ 7, 6 H ) . 13C-NMR (CDCl3): 136.0 (Br2C¼CH).
3,3-Diethoxy[2-13C]propyne (7c). The conversion of 18c to 7c employed the same procedure as that
described for the conversion of 18d to 7d (vide infra). The yield of 7c was 44% (0.417 g, 3.23 mmol). IR
(film): 3279m (HC ꢂ C), 2978m, 2932m, 2883m, 2076w (C ꢂ C), 1445w, 1329m, 1118s, 1056s, 1012m.
1H-NMR (CDCl3): 5.27 (dd, 2J(13C,H) ¼ 3.1, J ¼ 1.9, 1 H); 3.75 (dq, AB of ABX3, J ¼ 9.5, 7, 2 H); 3.59 (dq,
AB of ABX3, J ¼ 9.5, 7, 2 H); 2.55 (dd, J(13C,H) ¼ 49, J ¼ 1.9, 1 H); 1.25 (t, X3 of ABX3, J ¼ 7, 6 H ) .
2
13C-NMR (CDCl3): 79.0 (HC ꢂ C).
[2-13C]Propynal Tosylhydrazone (8c). The procedure for preparation of 8c is identical to that for 8b
(Procedure A, vide supra). As before, we were able to isolate the anti-isomer, anti-8c, whereas the syn-
isomer, syn-8c, was contaminated with the corresponding tosylpyrazole.
Data of anti-8c (0.133 g, 0.596 mmol, 18%). 1H-NMR (CDCl3): 8.03 (br. s, NH); 7.83 (m, 2 tosyl H);
7.34 (m, 2 tosyl H); 6.94 (dd, J(13C,H) ¼ 8, J ¼ 2.0, 1 H); 3.17 (dd, J(13C,H) ¼ 50, J ¼ 2.0, 1 H); 2.44 (s,
3 H). MS: 223 (Mþ, 46), 160 (11), 159 (91), 158 (11), 157 (12), 156 (17), 155 (100), 140 (35), 139 (25),
129 (14). HR-MS: 223.0498 (Mþ, C913CH10N2O2Sþ; calc. 223.0497).
2
2
Data of syn-8c (0.162 g, 0.726 mmol, 22%). 1H-NMR (CDCl3): 8.66 (br. s, NH); 7.72 (m, 2 tosyl H);
7.34 (m, 2 tosyl H); 6.62 (dd, 2J(13C,H) ¼ 10, J ¼ 1.0, 1 H); 3.77 (dd, 2J(13C,H) ¼ 50, J ¼ 1.0, 1 H); 2.44 (s,
3 H). 13C-NMR (CDCl3): 72.2.
Data for 1-Tosyl-1H-[4-13C]pyrazole (0.081 g, 0.363 mmol, 11%). 1H-NMR (CDCl3): 8.11 (dd,
2J(13C,H) ¼ 9, J ¼ 2.7, 1 H); 7.90 (m, 2 tosyl H); 7.73 (dd, 2J(13C,H) ¼ 11, J ¼ 1.5, 1 H); 6.39 (dd,
1J(13C,H) ¼ 178, J ¼ 1.5, 1 H); 2.42 (s, 3 H). 13C-NMR (CDCl3): 108.7.
3,3-Diethoxyprop-1-ene (19). Compound 19 was prepared from freshly distilled propenal (4.80 ml,
4.03 g, 1.8 mmol; Aldrich) and HC(OEt)3 (12.2 ml, 10.9 g, 73.3 mmol; Aldrich) catalyzed by TsOH
(0.0079 g, 0.042 mmol; Aldrich) according to the procedure of Dedieu et al. [54][55]. The acetal 19 was
isolated by reduced-pressure distillation (478/22 Torr) ([54][55]: b.p. 123 – 1258/760 Torr) in 74% yield
(d ¼ 0.837 g/ml). IR (film): 3085w (C¼CH), 1649w (C¼C). 1H-NMR (CDCl3): 5.86 (ddd, J ¼ 17.5, 10.5,
5.0, 1 H); 5.39 (ddd, J ¼ 1.0, 1.7, 17.5, 1 H); 5.28 (ddd, J ¼ 1.0, 1.7, 10.5, 1 H); 4.87 (td, J ¼ 1.0, 5.0, 1 H);
3.43 – 3.74 (m, 4 H); 1.15 (t, J ¼ 7.0, 6 H).
2,2-Diethoxyacetaldehyde (20). Compound 20 was prepared according to the procedure of Stetter and
Mohrmann [56 – 58]. A soln. of 19 (9.6 ml, 8.04 g, 61.7 mmol) in 60 ml of EtOH was cooled to ꢀ 788. A
stream of O3 in O2 was bubbled through the soln., until, it retained a light blue color, indicating excess of
O3, ca. 2 h. N2 was bubbled through the soln. for 10 min, and Me2S (5.12 ml, 4.31 g, 69.4 mmol) was added
dropwise. The soln. was warmed to 208 and stirred for 14 h. The product at this point is 1,1,2-
triethoxyethan-2-ol, the hemiacetal of 20. After removal of most of the EtOH via rotary evaporation, the
product was subjected to reduced-pressure distillation using a 30-cm Vigreux column. The first fraction is
EtOH, which is eliminated from the hemiacetal, leaving 20 in the pot. The second fraction comes over at
a head temp. of 45 – 508/0.5 Torr, containing a 7:1 mixture of 20 and its ethyl hemiacetal, and a trace of