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Chapter 31: Q32P. (page 1259)

The biosynthesis of lanosterol from squalene has intrigued chemists since its discovery. It is now possible, for example, to synthesize polycyclic compounds from acyclic or monocyclic precursors by reactions that form several C-C bonds in a single reaction mixture.

  1. Draw a stepwise mechanism for the following reaction.
  2. Show how X can be converted to 16, 17-dehydroprogesterone. (Hint: See Figure 24.5 for a related conversion.)

Short Answer

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A.

B.

Step by step solution

01

Biosynthesis reaction

Biosynthesis reactions involve complex structures in living organisms, and larger rings are synthesized from smaller rings by the ring-expansion reactions.It involves oxidative cleavage of alkenes and intermolecular aldol reactions.

02

Stepwise mechanism for the reaction

(a) A stepwise mechanism involves the reaction with water and forming a carbocation. Again, on reaction with water, the required compound is formed.

The stepwise mechanism for the reaction

03

Conversion of X to 16,17-dehydroprogesterone

(b) X compound underwent oxidative cleavage of an alkene with ozone (O2), followed by Zn, H2O. After that, an intermolecular aldol reaction occurs in the dilute OH- and H2O; it forms 16,17-dehydroprogesterone.

Conversion of X to 16,17-dehydroprogesterone

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Most popular questions from this chapter

(a) Draw a skeletal structure of the anabolic steroid methenolone from the following description. Methenolone contains the tetracyclic steroid skeleton with a carbonyl group at C3, a hydroxyl group at C17, a double bond between C1 and C2, and methyl groups bonded to C1, C10 and C13. (b) Add wedges and dashed wedges for all stereogenic centers with the following information: The configuration at C10 is R, The configuration at C13 is S, the configuration at C17 is S, and all substitutents at ring fusions are trans to each other. (c) Draw the structures of Primobolan, the product formed when methenolone is treated with CH3(CH2)5COCl and pyridine. Primobolan is an anabolic steroid that can be taken orally or by injection and has been used illegally by well-known Major League Baseball players.

A difficult problem in the synthesis of PGF2αis the introduction of the OH group at C15 in the desired configuration.

  1. Label this stereogenic center as R or S.

  2. A well known synthesis of PGF2αinvolves reaction of A with Zn((BH4)2a metal hydride reagent similar in reactivity toNaH4, to form two isomeric products, B and C. Draw their structures and indicate their stereochemical relationship.

  3. Suggest a reagent to convert A to the single stereoisomer X.

Convert each ball-and-stick model to a skeletal structure that clearly shows the stereochemistry at the ring fusion of these decalin derivatives.

A.

B.

Draw three-dimensional structures for each alcohol. Label the OH groups as occupying axial or equatorial positions.

a.

b.

c.

d.

a. Draw a three-dimensional structure for the following steroid.

b. What is the structure of the single stereoisomer formed by reduction of this ketone with H2 , Pd-C? Explain why only one stereoisomer is formed.
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