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Selective reduction of the nitro group in 8 presented another major challenge. Conditions such as Zn,1 Zn/HC02NH4, Zn/H2NNH2.HC02H,17 In,18 In/NH4C1,19 Fe,20 Fe-FeCl*21 Mg/H2NNH2-HC02H,22 and SnCl^ as well as Adam's and Lindlar’s catalysts, either led to over-reduction or to no reaction. Transfer hydrogenation using HC02H or H2 over 1096 Pd/C led to over-reduction. On the other hand, use of HC02NH4 with either 10% Pd/C23 or RaNi returned the starting material. Remarkably, HC02H with catalytic RaNi gave diastereomeric amine 9 in 88% yield (Figure 7, structure by X-ray).
Conversion of amine 9 into racemic core 10 (D-ring) was accomplished by an intramolecular reductive amination
between Cl6 and the nitrogen via in situ hydrolysis of the acetal group using 0.5 N aqueous HC1 at 100 °C followed by formation of an iminium ion and its reduction with NaCNBH3 into a racemic mixture of amines 10 (8,14dihydronorsalutar- idine and norisosinomenine). No hydrolysis of the methyl enol ether occurred under these conditions. A yield of 70% was obtained for 10 which upon treatment with 37% H2CO(aq), NaCNBH3, and AcOH in EtOH25 led to N-methylation to give 11 as a racemic mixture of isosinomenine and 8,14* dihydrosalutaridine in 91% yield (Figure 8, structure by X-ray).
The strategy presented here is an efficient (17% from 1, 10 steps) racemic synthesis of the aforementioned 8,14-dihydro- morphinandienone alkaloids. The synthesis furnishes the correct oxidation level of the target compounds by avoiding the biomimetic strategy of o,p-phenolic coupling. A unique route to a Henry—Michael—dehydration cascade has been developed where a variety of other conditions failed. The resultant aliphatic nitro group could be reduced selectively only under transfer hydrogenation conditions. The current strategy is complementary to that from our group for the synthesis of codeine9 in that it begins with the C6 oxygen already in place, obviating the need for its later introduction over several steps. A one-step conversion of 7 to 9, a one-pot conversion of 9 to 11, and an asymmetrical conversion of 5 to 6 are topics for future consideration. Conversion of 8,14-dihydrosalutaridine into salutaridine (i.e., oxidation of C8—C14 into a double bond as exemplified by conversion of ocobotrine into sinoacutine via cr-bromination-dehydrobromination a) would allow for its subsequent transformation into thebaine26 and codeine,27 providing a practical synthesis of these iconic structures.