Methenylated compound

The active species in the Tebbe olefination is believed to be the nucleophilic (Schrock-type) titanocene methylidene, which is formed from the Tebbe reagent upon coordination of the aluminum with a Lewis base (e.g., pyridine). This methylidene in its uncomplexed form, however, has never been isolated or observed spectroscopically owing to its extreme reactivity. The same intermediate can also be generated by other means." The titanocene methylidene reacts with the carbonyl group to form an oxatitanacyclobutane intermediate that breaks down to titanocene oxide and the desired methenylated compound (alkene). The driving force is the formation of the very strong titanium-oxygen bond. 

Homofolic acid, 5,11-methenyl-tetrahydro-biological activity, 3, 327 Homofolic acid, tetrahydro-biological activity, 3, 327 Homoisoflavanones occurrence, 3, 722 thermoisomerization, 3, 722 thermolysis, 3, 728 Homolytic reactions heterocyclic compounds reviews, 1, 74 Homophthalic acid isocoumarins synthesis from, 3, 830 synthesis, 3, 830 Homophthalic anhydride isochroman-l-one synthesis from, 3, 860 20a-Homoporphyrin nomenclature, 1, 30 Homopterocarpin isolation, 4, 998 ( )- D- Homotestosterone synthesis, 1, 453 Homer-Emmons reaction chromene synthesis by, 3, 749 Hortiacine isolation, 3, 149 Hortiamine isolation, 3, 149... 

The porphyrins found in nature are compounds in which various side chains are substituted for the eight hydrogen atoms numbered in the porphin nucleus shown in Figure 32-1. As a simple means of showing these substitutions, Fischer proposed a shorthand formula in which the methenyl bridges are omitted and each pyrrole ring is shown as indicated with the eight... 

One-carbon units in different oxidation states are required in the pathways producing purines, thymidine, and many other compounds. When a biochemical reaction requires a methyl group (methylation), S-adenos dmethionme (SAM) is generally the methyl donor. If a one-carbon unit in another oxidation state is required (methylene, methenyl, formyl), tetrahydrofolate (THF) typically serves as its donor. 

Tetrahydrofolic acid then functions as a carrier of one-carbon groups for amino acid and nucleotide metabolism. The basic ring system is able to transfer methyl, methylene, methenyl, or formyl groups, and it utilizes slightly different reagents as appropriate. These are shown here for convenience, we have left out the benzoic acid-glutamic acid portion of the structure. These compounds are all interrelated, but we are not going to delve any deeper into the actual biochemical relationships. 

Fig. 12. A traditional example of an acyclic carbon compounds and its IUPAC name 3(2-propynyl),5-methenyl-oct-1,2-diene,6-yne...

Tetrahydrofolate (THF) is the major source of 1-carbon units used in the biosynthesis of many important biological molecules. This cofactor is derived from the vitamin folic acid and is a carrier of activated 1-carbon units at various oxidation levels (methyl, methylene, formyl, formimino, and methenyl). These compounds can be interconverted as required by the cellular process. The major donor of the 1-carbon unit is serine in the foUowing reaction ... 

METHENYL TRIBROMIDE (75-25-2) Violent reaction with chemically active metals, acetone, calcium, strong caustics, potassium, potassium hydroxide. Increases the explosive sensitivity of nitromethane. Incompatible with crown polyethers, sodium-potassium alloys. Forms friction- and shock-sensitive compounds with lithium. Aqueous solution is a medium-strong acid. Liquid attacks some plastics, rubber, and coatings. Corrosive to most metals in the presence of moisture. 

This methenylation is accompanied by the salification of the acidic SO3H group by the basic hexamine. It is worthy of note that the NHg group in sulfamic acid behaves here like the so-called occult amine group in organic compounds. 2... 

Formation of a lV -methenyl bridge (in other terms, an imidazolidine ring) by removal of water accounts for the stability of the enzymatic hy-droxymethyl-FH4 product the synthetic compound is sometimes called anhydroleucovorin. 

As shown in Figure 11.19, tetrahydrofolate can carry one-carbon fragments attached to N-5 (formyl, formimino or methyl groups), N-10 (formyl) or bridging N-5—N-10 (methylene or methenyl groups). 5-Formyl-tetrahydrofolate is more stable to atmospheric oxidation than folate itself, and is therefore commonly used in pharmaceutical preparations it is also known as folinic acid, and the synthetic (racemic) compound as leucovorin. 

When formiminoglycine (FIG) was broken down to glycine, derivatives of folic acid were involved and ATP formation occurred (H, 297). These reactions are shown in Fig. 17. The first of these reactions was catalyzed by FIG formimino transferase FIG reacted with FH4, free glycine was formed, and iV -formimino-FH now carried the formimino grouping. In the second step, NH3 was released from iNT -formimino-FEU and the cyclic iVs. Vio-imidazolinium derivative of iV -formyl-FH4 resulted. The latter compound was also called lV ,Ari >-methenyl-FH4, anhydroleucovorin, or anhydrocitrovorum factor the enzyme responsible for the formation of the imidazolinium derivative from A -formimino-FH4 was named formimino-FH4 cyclodeaminase. In the third reaction, methenyl-FH4 cyclohydrolase catalyzed the conversion of lV ,iW -methenyl-FH4 to JV -formyl-FH4. The fourth step, which resulted in ATP formation, was catalyzed by FH4-formylase. ADP and P reacted with JW -formyl-FH4 and produced FH4, free formic acid, and ATP. The reverse of this reaction resulted in the activation of free formic acid [Eq. (5)]. 

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