Y-group

The protecting group Y of the amine is generally an alkoxycarbonyl derivative since their nucleophilicity is low. Benzyloxy- or tert-butoxycarbonyl derivatives usually do not undergo azlactone formation. 

Berberine is probably the most widely distributed alkaloid. It and the allied alkaloids palmatine, jatrorrhizine, columbamine and coptisine occur somewhat frequently in the Rhcnadales (list, p. 169) as the tetrahydro-derivatives, but, in the botanical families referred to in the distribution list below, the tetrahydro-derivatives are exceptional and the unreduced alkaloids usual. The associated alkaloids include two members of the aporphine group, domesticine and t odomesticine (p. 315), one member of the cryptopine group, y-homochelidonine (p. 294) and two members of the double woquinoline type, viz., berbamine and oxyacanthine (p. 346). 

The efficacy of ring fluorination depends on the nature and position of the activating group, Y, m the aromatic ring The relative extent of for N (CH3)3 displacement decreases in the following order for Y p-NOy (71%), p-CN (24%), P-CH3CO (15%), p CHO (<5%) = m NO2 (<5%) This fluorodequaternization technique was subsequently adapted to prepare numerous NC A (no-camer-added) F-labeled aryl fluorides [7J, 74]... 

The difference in behavior between aldehydes/ketones and carboxylic acic derivatives is a consequence of structure. Carboxylic acid derivatives have ai acyl carbon bonded to a group -Y that can leave as a stable anion. As soon a the tetrahedral intermediate is formed, the leaving group is expelled to general- a new carbonyl compound. Aldehydes and ketones have no such leaving grouj however, and therefore don t undergo substitution. 

We consider an extrusion reaction to be one in which an atom or group Y connected to two other atoms X and Z is lost from a molecule, leading to a product in which X is bonded directly to Z. 

In a rearrangement reaction, a group moves from one atom to another in the same molecule. Most are migrations from an atom to an adjacent one (called 1,2 shifts), but some are over longer distances. The migrating group (Y) may move with its... 

In any rearrangement we can in principle distinguish between two possible modes of reaction In one of these, the group Y becomes completely detached from A and... 

X = electron-donating group Y = electron-withdrawing group... 

Scheme 39 Regioselectivity predicted on the basis of matching of the nucleophiUcity of the diene having electron-donating group X and the electrophiUcity of the dienophile having electron-with drawing group Y...

Fig. 8. Relative reactivities of different entering groups Y with PtdienBr in aqueous solution at 25 °C. Data from ref. 74.

Table 4. Synthesis of optically active organotin compounds RR R"SnL from an optically unstable compound bearing a chiral leaving group Y RR R"Sn—Y RR R"Sn-L ... 

The El/S l ratio is, of course, substantially independent of the leaving group Y, but this is not the case with E2/Sjv2, where breaking of the C—Y bond is involved in each alternative T.S. The following rough sequence, in order of increasing promotion of elimination, is... 

The more powerfully electron-withdrawing the substituent the greater the chance that the T.S. in an E2 elimination will be carbanion-like cf. p. 257), or even that the reaction pathway may be shifted to the ElcB mode (cf. p. 249), e.g. possibly with N02 or ArS02, especially if the leaving group, Y, is a poor one. 

The reaction of rans-[IrCl(CO)(P(p-C6H4Y)3)2] with H2 at 30 °C depends on the nature of the group Y in the phenyl group. For several ligands, the AH and AS vary as follows ... 

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