5. Because of the product's volatility, a
hot water bath should not be used during solvent evaporation. GC and GC/MS analysis of an aliquot indicate that the product ranges in purity from 75–95% with unreacted
1-iodoheptane also present. The addition of
0.5 equiv of hexamethylphosphoramide (HMPA) prior to addition of the
iodoheptane was found to improve the yield of this alkylation. The addition of
0.5 to 2.0 equiv of 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU) did not improve the yield. The checkers found the following GC conditions useful for monitoring the alkylation reaction: initial column temperature, 40°C; heating increment, 10°C/min;
iodoheptane R
f = 3.3 min, product R
f = 5.7 min. Column specifications were as follows: SPB-1 (stationary phase), fused
silica gel capillary column, 30 m × 0.32 mm ID, 0.25-mm film thickness.
6.
Acetone is commonly used as a solvent in Jones oxidations; however, the desired keto acid tends to be retained by the chromium salts during work-up. A benzene/THF solution has also been employed for the oxidation, but this modification did not seem to have much effect on the overall yield.
11. The product may be recrystallized from hexanes to mp
79–80°C. The spectral data of the recrystallized material are as follows:
1H NMR (500 MHz, CDCl
3) δ: 0.86 (t, 3 H, J = 7.0), 1.30 (s, 8 H), 1.57 (quintet, 2 H, J = 7.3), 2.43 (t, 2 H, J = 7.5), 2.61 (dd, 2 H, J = 6.23, 5.87), 2.71 (dd, 2 H, J = 6.60, 6.23);
13C NMR (125 MHz, CDCl
3) δ: 14.0, 22.6, 23.8, 27.7, 29.0, 29.1, 31.6, 36.7, 42.7, 178.5, 208.9; IR (thin film) cm
−1: 3056, 2987, 2957, 2933, 2873, 2858, 1710, 1421, 1265, 748. Other electrophiles were used to give the corresponding keto acids as shown below: