Chemistry Reference
In-Depth Information
H
C D
H
D
C
B
A
A B
H
H
H
H
H
all-
trans
H
O
coplanar bonds
all-
trans
O
put in substituents
H
O
H
D
C
axial
H
H
D
B
C
A
H
O
B
H
A
6
O
H
H
O
H
H
ring A distortion because of coplanarity
associated with unsaturated ketone
The steroid skeleton in question differs from the all-
trans
-fused system only in respect of ring A, which has
an unsaturated ketone. Because of the coplanarity of bonds in this system, there will be some distortion in ring
A, which can be represented (approximately) as shown.
We then need to put in the substituents. The critical one to note is the methyl in ring B, because this must be
axial. In the following part of the question, the stereochemistry at this position changes, so that the methyl becomes
equatorial. This is not a conformational change, but a configurational change, so requires appropriate chemistry.
We should be formulating a proposal where this centre first becomes planar, and then reverts to tetrahedral, but
with the substituent then taking up the more favourable equatorial position.
O
protonation of carbonyl
H
enolization through
conjugated system
O
H
H
H
6
H
O
H
H
H
H
HO
H
H
O
O
H
O
H
H
H
H
H
H
H
6
H
O
H
O
6
H
H
HO
enol tautomer
reversion to keto tautomer allows proton to be
acquired from upper face, so that substituent
takes up favourable equatorial position
The functional group in ring A is a ketone; a planar system that takes in carbons adjacent to a ketone is the
enol tautomer. We are using acid catalysis here, and that is appropriate for enolization. If we first protonate the
carbonyl, then try moving electrons, we shall soon find that the proton at position 6 (standard steroid numbering)
can be lost in generating a conjugated enol tautomer. In a reversal of this process back to the ketone, we can
pick up a proton at position 6 from either face. In this case, protonation on the upper face allows the methyl
substituent to take up the more favourable equatorial position.
