Here are some chemical suppliers that are highly regarded for small-scale custom synthesis services:
1. SynHet
Services: Offers cCRO (chemical Contract Research Organization) and FTE (Full-time equivalent) services for small-scale synthesis of non-commercially available compounds on an exclusive basis. They provide fully integrated and standalone chemistry solutions from hit identification, lead optimization to early chemical development for biotechnology, pharmaceutical industries, and academic institutes. They also support research activities for agrochemical and material science industries.
Advantages: ISO9001 quality system ensures high-quality production of target molecules. They offer flexible pricing and quick project communication (usually within 12 hours).

2. Veeprho
Services: Veeprho Group offers global custom synthesis services to pharmaceutical and biotech chemical suppliers. They are project-based, flexible, cost-effective, and committed to exceeding customer expectations. They have extensive experience in multi-step synthetic product development and can support custom synthesis from the R&D lab to commercial scale.
Technical Expertise: Their team has wide experience in working on a variety of complex chemical reactions and can deliver high-quality custom-based compounds from milligrams to kilograms. They have developed over 1000 projects and delivered them to clients in a cost-efficient way.

3. Chemleader
Services: Chemleader provides custom synthesis of small molecules for applications across the pharmaceutical, life science, and fine chemical sectors. They offer detailed reports and support for all products.

4. Richman Chemical
Services: Richman Chemical has been providing custom chemical synthesis and manufacturing services since 1988. They can handle projects from proof of concept, initial gram quantities, scale-up processing to regular commercial quantities. They meet various regulatory requirements and handle a wide range of custom chemical synthesis challenges.
Advantages: One-stop shopping experience with experienced project managers.

5. Life Chemicals
Services: Life Chemicals offers custom synthesis services for a variety of small-molecule compounds, including building blocks, starting materials, intermediates, drug-like screening compounds, compound library synthesis, derivatives of lead compounds, and more. They have a skilled and experienced team of organic chemists who can confidently and punctually carry out custom synthesis.
Advantages: Development of novel and customized synthetic routes, optimization of cost- and time-efficient synthetic routes, and alternative synthetic routes to avoid hazardous materials or inappropriate methods.

6. OTAVA Chemicals
Services: OTAVA Chemicals offers custom synthesis of organic molecules for biotech and pharmaceutical applications, as well as design and synthesis of fluorescent dyes with requested characteristics. They have small-scale and large-scale chemistry capabilities and can undertake projects from idea to final production at any scale.
Advantages: Operational flexibility, long-standing expertise, and the ability to incorporate cheminformatics, molecular modeling, and computational chemistry procedures.

7. BOC Sciences
Services: BOC Sciences provides contract research, development, and manufacturing services to help customers industrialize their products from research to clinical trials to commercialization. They offer lab scale custom synthesis, biosynthesis, bioconjugation, and analytical services.
Specializations: Chiral synthesis, nucleosides and nucleotides synthesis, heterocyclic compounds synthesis, isotope chemical synthesis, peptide synthesis, and bioconjugation.

8. Hong Jin Chemical
Services: Hong Jin has a research and development laboratory and pilot plant. They have various equipment for small-scale and large-scale production and can produce a variety of chemicals.

9. XinChem
Services: XinChem is an active and professional custom synthesis and contract manufacturer in China since 2005. They are committed to producing and supplying high-quality intermediates, functional chemicals, peptides, fine chemicals, additives, coatings, resins, etc.

10. ChiroBlock
Services: ChiroBlock offers small-scale custom synthesis services and has built strong partnerships with other chemical suppliers in this field, providing large-scale manufacturing and other related services.

11. AChemBlock (Advanced ChemBlocks Inc)
Services: AChemBlock offers custom synthesis services for a variety of organic compounds from milligram to kilogram scales at competitive prices.

12. Raybow
Services: Raybow is committed to delivering top-tier custom synthesis and contract research services, designed to fast-track your internal research and development initiatives. They offer precise and excellent services to support drug development.

13. ChemPacific Corporation
Services: ChemPacific's custom chemical synthesis business offers expertise in developing and manufacturing organic chemicals with small-company flexibility. They routinely scale up from milligrams to kilograms.

These chemical suppliers are known for their ability to handle small-scale custom synthesis projects with high quality and efficiency. If you have specific requirements or need more detailed information, it is recommended to contact these chemical suppliers directly.