Synthetic Conversions

Subsequently, a study of the synthetic route corresponding to the direct disconnection of ring C, 263 => 271 was reported. Although the critical synthetic conversion of 271 to 263 showed only modest stereoselectivity (c<3. 3 1), the synthetic sequence is convergent. 

Indicate reaction conditions or a series of reactions that could effect each of the following synthetic conversions ... 

Several synthetic transformations that are parts of total syntheses of natural products are summarized by retrosynthetic outlines. For each retrosynthetic transform suggest a reagent or short reaction sequence that could accomplish the forward synthetic conversion. The proposed route should be diastereoselective but need not be enantioselective. 

Whereas we might in principle have a catalyst available today for every synthetic conversion, it will probably take another 10-20 years before enjoying the full potential of cascades of catalysis on an industrial scale (Fig. 13.18). 

Some synthetic transformations are shown in the retrosynthetic format. Propose a short series of reactions (no more than three steps should be necessary) which could effect the synthetic conversion. 

Undesirable intermolecular reactions can be avoided during certain synthetic conversions. Thus it is often useful to carry out C-alkylation and C-acylation of compounds that form enolate anions, for example, esters with a-hydrogens. Such reactions are often complicated by self-condensation since the enolate anion can attack the carbonyl group of a second ester molecule. Attachment of the enolizable ester to a polymer support at low loading levels allows the alkylation and acylation reactions (Eq. 9-79) to be performed under... 

A-7. Using any necessary organic or inorganic reagents, provide the steps to carry out the following synthetic conversion ... 

A-3. Outline a sequence of steps that would allow the following synthetic conversions to be carried out ... 

Several compounds of this type, formally derived from aldoses by oxidation of the terminal CH2OH group to -CHO, have been prepared. Dialdoses arise as intermediates in structural studies, but they are also valuable starting materials for synthetic conversions, in particular for natural-product synthesis. A branched-chain dialdose, streptose (167), occurs as a component of the antibiotic streptomycin. The structure of streptose was elucidated after extensive investigations of its derivatives and transformation products.371 The 2,5-dimethoxytetrahydrofuran fulvanol (168), an analogue of apiose, has been isolated from the plant Hemerocallis fulva 12... 

As discussed in Chapter 1, many electrophiles are stable in solvents and the presence of substances of moderate Lewis basicity. Even relatively strong electrophiles can be generated and handled under appropriate conditions.1 A wide variety of electrophilic carbocationic and onium ion salts have been prepared and characterized by spectroscopic methods, X-ray crystallography, kinetic studies, and other techniques. Several types of these electrophilic salts are even available commercially (such as varied onium, iminium, and carbocationic salts, etc.). Electrophiles are generally generated by the reactions of suitable precursors with Brpnsted or Lewis acids. They can be sometimes isolated, but for synthetic conversions they are more often generated in situ and reacted directly with nucleophilic reagents. 

Superacidic FSO3H (fluorosulfonic acid, Ho — 15) has also been used in some studies involving superelectrophilic activation. However, due to its tendency for sulfonation and oxidation, this acid has found only limited use in synthetic conversions involving superelectrophiles. Fluorosulfonic acid has been shown effective to activate nitronium salts in their reactions with weak nucleophiles, and again it was suggested that the protosolvated species (6) is involved in the reactions.28 Both fluorosulfonic acid and triflic acid have been reported to give the diprotonated species (14) from 3-arylindenones (eq 12) 29... 

Superelectrophilic reactions have been shown to be particularly useful in various synthetic conversions particularly of unactivated er and 7r-bonds, such as in alkanes and electron deficient arenes. Superelectrophiles have also been used in the synthesis of natural products and biologically active compounds. Superelectrophilic chemistiy has also... 

Since the concept of superelectrophilic activation was proposed 30 years ago, there have been many varied superelectrophiles reported both in experimental and theoretical studies. Superelectrophiles can be involved in both gas and condensed phase reactions, ranging from interstellar space down to the active sites of certain enzymes. Moreover, synthetic conversions involving superelectrophiles are increasingly used in the synthesis of valuable products. Superelectrophilic activation has also been useful in the development of a number of new catalytic processes. It is our belief that superelectrophilic chemistry will continue to play an increasing role in both synthetic and mechanistic chemistry. 

Show how you would accomplish the following synthetic conversions, (a) Convert 1-methylcyclohexene to 1-bromo-l-methylcyclohexane. 

Show how you would accomplish the following synthetic conversions. 

A novel tandem photolysis was observed for the synthetic conversion of 2,3-diphenylbenzo[. ]furan to benzo[. ]-phenanthro[9,10-, furan 134. This process might involve sequential photochemical cyclization and aerial oxidation (Scheme 60) <2003TL3151>. 

The 3, 5-cis configuration was assigned by NOE experiments and confirmed by synthetic conversion into S R /tyS -dihydroxyproline (34), an amino acid constituent of virotoxin that can be easily obtained from the (3S, 4R,5S)-bromolactone. Thus, the y-lactone is treated with trifluoroacetic acid to remove the A+protecting group, then hydrolysis with 0.5N sodium hydroxide provides the dihydroxyproline 34 in 60% yield. 

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