B-ring

The reaction has been applied successfully to the synthesis of a precursor of provitamin D. 520, which has a homoannular conjugated diene in the B ring[340]. Treatment of the 7a-carbonate 518 with Pd cataJy.st at 40 C afforded the 5.7-diene 520 regioselcctively in good yield. No heteroannular diene 521... 

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

The viscosity of a straight-mn or fluxed-back pitch can be calculated from the R-and-B (ring-and-ball) softening point ... 

The vitamins D ate 9,10-secosteroids, that is, steroid molecules with an opened 9,10 bond of the B-ring. The relationship between the provitamin steroid (pethydto-l,2-cyclopentanophenanthrene ring system) and the 9,10-secosteroid nucleus is shown in stmctures (5) and (6), cholestane and 9,10-secocholestane (calcitane), respectively. 

The A-ring of the 17-ol (25) derived from equilenin 3-methyl ether is reduced rapidly under Birch reduction conditions, since the 1,4-positions are unsubstituted. The B-ring is reduced at a much slower rate, as is characteristic of aromatic compounds in which 1,4-reduction can occur only if a proton enters an alkylated position. Treatment of (25) with sodium and t-butyl alcohol in ammonia reduces only the A-ring to afford the corresponding 1,4-dihydro compound in over 85% yield.On the other hand,... 

Reduction of the sodium salt of equilenin 17-ethylene ketal with lithium, sodium or potassium in ammonia at —70° occurs predominantly in the B-ring, affording, after acid hydrolysis, equilin (29) in up to 76% yield (55% isolated). The preferential reduction of the B-ring reflects the relative, but not absolute, resistance to reduction conferred on the A-ring by the naphthoxide ion. Some A-ring reduction does compete kinetically with B-ring reduction, since the epimeric 3-hydroxyestra-5,7,9-trien-17-ones are the major reaction by-products. Simple phenoxide ions usually reduce slowly... 

A- and B-ring double bonds are simultaneously reduced in the last step. 

Brassard " applied the Conrad-Limpach reaction as an approach to the A-B ring system of Phomazarin. While the overall yield was only 23%, he showed that a methoxy group was an acceptable substituent on enamino-ester 46 and for the subsequent cyclization to quinolone 47. 

The first reaction series to be considered are the basicities of the various quinolines. Baciocchi and Illuminati have demonstrated that the pK values of quinolines substituted in the B-ring follow the Hammett equation well if ApK, i.e., the difference between the pK values of substituted and unsubstituted compounds, is plotted against a, the quinoline points fall on the same line as the pyridine points, as shown in Fig. 5, so that the p-values for the two series are identical. 

INOC has been used for the synthesis of tncylic compounds havmg the taxane A/B ring system with an aromatic C nrg fEq. 8.68. ° ... 

Figure 27.11 Steroid conformations. The three six-membered rings have chair conformations but are unable to undergo ring-flips. The A and B rings can be either cis-fused or trans fused.

Steroid, 1079-1089 adrenocortical, 1083 anabolic. 1083 androgens, 1082 biosynthesis of, 1084-1089 cis A-B ring fusion in, 1081 conformation of, 1081... 

As we will see later in the synthesis, both groups adopted the same strategy for the synthesis of the B-ring of cobyric acid (4). 

Scheme 8 presents the sequence of reactions that led to the synthesis of the B-ring of vitamin B12 by the Eschenmoser group. An important virtue of the Diels-Alder reaction is that it is a stereospecific process wherein relative stereochemical relationships present in the diene and/or the dienophile are preserved throughout the course of the reaction.8 Thus, when the doubly activated dienophile 12 (Scheme 8) is exposed to butadiene 11 in the presence of stannic chloride, a stereospecific reaction takes place to give compound 27 in racemic form. As expected, the trans relationship between... 

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