The Zakarian Synthesis of ( + )-Pinnatoxin A

( + )-Pinnatoxin A 3, isolated from the shellfish Pinna muricata, is thought to be a calcium channel activator. A key transformation in the synthesis of 3 reported (J. Am. Chem. Soc . 2008, 130, 3774) by Armen Zakarian, now at the University of California, Santa Barbara, was the diastereoselective Claisen rearrangement of 1 to 2. The alcohol portion of ester 1 was derived from the aldehyde 4, prepared from D-ribose. The absolute configuration of the secondary allylic alcohol was established by chiral amino alcohol catalyzed addition of diethyl zinc to the unsaturated aldehyde 5. The acid portion of the ester 1 was prepared from (S)-citronellic acid, by way of the Evans imide 7. Methylation proceeded with high diasterocontrol, to give 8. Functional group manipulation provided the imide 9. Alkylation then led to 10, again with high diastereocontrol. In each case, care had to be taken in the further processing of the α-chiral acyl oxazolidinones. Direct NaBH4 reduction of 8 delivered the primary alcohol. To prepare the acid 10, the alkylated acyl oxazolidinone was hydrolyzed with alkaline hydrogen peroxide. On exposure of the ester 1 to the enantiomerically-pure base 11, rearrangement proceeded with high diastereocontrol, to give the acid 2. This outcome suggests that deprotonation proceeded to give the single geometric form of the enolate, that was then trapped to give specifically the ketene silyl acetal 12. This elegant approach is dependent on both the ester 1 and the base 11 being enantiomerically pure. The carbocyclic ring of pinnatoxin A 3 was assembled by intramolecular aldol condensation of the dialdehyde 11. This outcome was remarkable, in that 11 is readily epimerizable, and might also be susceptible to β-elimination. Note that the while the diol corresponding to 11 could be readily oxidized to 11 under Swern conditions, attempts to oxidize the corresponding hydroxy aldehyde were not fruitful.

Adriamycin impairs the contraction of mesangial cells through the inhibition of protein kinase C and intracellular calcium

The effects of adriamycin on the contractile function of cultured mesangial cells were measured by the changes in planar surface area in response to treatment with agonists. Incubation of mesangial cells with adriamycin (0.2 μg/ml) for 24 h significantly decreased the contractile responses to the calcium channel activator BAY K 8644 (1 μM) and to the PKC activator PMA (1 μM). Intracellular calcium concentration ([Ca2+]i), measured by changes in fura 2 levels in response to ATP (0.1 mM), was significantly inhibited in adriamycin-treated mesangial cells compared with control cells. In the absence of extracellular calcium, treatment with ionomycin (0.1 mM) or thapsigargin (10 μM) resulted in a significantly smaller increase in [Ca2+]i in adriamycin-treated mesangial cells compared with control, suggesting an important role of the endoplasmic reticulum in the effects of adriamycin. Using PKC-specific antibodies, adriamycin significantly decreased the cytosolic and membranous fractions of PKC-α in mesangial cells to 75 ± 6 and 70 ± 12% of control, respectively. The PKC activity of mesangial cells was also significantly inhibited after incubation with adriamycin for 24 h. In conclusion, adriamycin induces hypocontractility of mesangial cells, which may mediate this effect by inhibiting PKC-α and [Ca2+]i.

Calcium channels contribute to the decrease in blood pressure of pregnant rats

Pregnancy is associated with hemodynamic changes such as reduced vascular resistance and blood pressure. We reported that, during late pregnancy, the activity of voltage-dependent calcium channels (VDCC) is altered in the adrenal cortex and vascular smooth muscle. These observations suggested that the late pregnancy-induced decrease in blood pressure is linked to diminished VDCC function. We attempted to prevent pregnancy-induced reduced blood pressure with a calcium channel activator (CGP 28392) in pregnant rats and to mimic it by administration of a calcium channel blocker (nifedipine) to nonpregnant rats. Treatment was given from the 15th day of gestation for 7 days. The systolic blood pressure of CGP 28392-treated pregnant rats rose transiently for 2 days and then declined toward values of nontreated pregnant controls, although remaining higher. However, nonpregnant rats maintained their high arterial pressure throughout CGP 28392 treatment. Nifedipine lowered the blood pressure in nonpregnant rats to values of nontreated term-pregnant controls. Both agents did not affect body weight, water or food intake, plasma renin activity, and plasma aldosterone or corticosterone levels. Nifedipine and CGP 28392 treatment of nonpregnant and pregnant animals, respectively, did not modify the response of aortic rings to KCl. These results show that VDCC activation caused hypertension, which modified the extent of the decrease in blood pressure at the end of pregnancy.