Regioselectivity, catalytic hydrogenation

In principle, the direct hydride addition or catalytic hydrogenation, which did not give chlorins, was replaced by an electrocyclic intramolecular addition which is much easier with the above system. Complete regioselectivity was also achieved since electrocyclization did not occur with the resonance-stabilized ring C. 

Another example, in which the piperidine cycle is generated de novo, exploits a hetero Diels-Alder cycloaddition of 1 -/r-tolylsulfinyl-1,3-penta-diene 91 with benzylnitrosoformate, that generates an oxazine 92 with complete regioselectivity and 7i-facial diastereoselectivity.69 Osmilation of the double bond inserts stereoselectively two hydroxyl groups on the oxazine skeleton, protection and catalytic hydrogenation finally afforded the enantiomerically pure imino sugars 94 (Fig. 38). 

Removal of the silyl protecting group using TBAF, and subsequent acetylation, led to the diacetate 818. Regioselective nitration and catalytic hydrogenation provided the (R)-arylamine 814 (613) (Scheme 5.95). 

Catalytic hydrogenation of (R,S)-22 was unsatisfactory for removal of the chiral auxiliary. The regioselectivity of the TV-benzylation step (5% Pd-C, 5 bar H2, 40°C, EtOH, AcOH) gave a (S)-l-a m i ik > - in da n e/( W) - PG A ratio of 4 6. This means that 60% of the key intermediate was converted to the undesired product. Therefore, the retro-Strecker method, analogous to the homoallylamines case, was applied as an alternative. The initially 3-step method could be improved to an efficient one-pot procedure (Scheme 25.9). 

It should not surprise you that regioselective reduction of the C=C double bond alone is best done using catalytic hydrogenation as the C=C bond is weaker than the C=0 bond. The flavouring compound known as raspberry ketone is made by this method. 

The key step in the synthesis of 2-deoxycrustecdysone from the corresponding 20-oxo steroid is the stereoselective catalytic hydrogenation of the a,(3-unsaturated lactone to afford a 2 3 mixture of 8- and "y-lactones, respectively. This crude product was converted into the thermodynamically more stable y-lactone by treatment with aqueous NaOH. a,p-Unsaturated aldehydes and ketones are hydrogenated to produce allylic alcohols in high regioselectivity using hydridoiridium phosphine complexes, such as [Ir(PEt2Ph)4]4. 5 ... 

Catalytic hydrogenation of cis N-Boc aziridine 2-phosphonates (329) derived from 3-amino-2-hydroxyalkyl phosphonates affords N-Boc a-amino-2-phos-phonic esters (330) regioselectively (Figure 51). " ... 

Regioselective de-O-benzylation. An alternative strategy to partially benzylated carbohydrates has been accomplished by selective de-O-benzylation of easily available polybenzylated precursors. This has been achieved in limited cases by catalytic hydrogenolysis [37], catalytic hydrogen-transfer cleavage [38], acetolysis [39], hypoiodite fragmentation [40], iodine-mediated addition-elimination sequences [41], or use of Lewis acids [42]. 

The regioselective catalytic hydrogenation of 40 afforded a mixture of D-amhmo-2-hexulosonic acid (41) and L-act/Zo-2-hexulosonic acid (23) with 41 predominating (75,76). This result is in complete accord with the proposed solution structure for 40 (40a) because catalytic hydrogenation of cyclohexanones affords predominantly axial hydroxyl groups. 

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