---
title: "Python Retrosynthesis Package: RENKIN Without RDKit"
description: "How to plan multi-step retrosynthesis routes in Python with RENKIN -- a pip-installable, RDKit-free, pure-Rust engine with custom stock, templates, and evidence metadata."
---
# Python Retrosynthesis with RENKIN
If you're looking for an open-source Python library for computer-aided
synthesis planning (CASP) that doesn't require RDKit or a C/C++ toolchain,
RENKIN ships as a pip-installable wheel with the search engine, template set,
and building-block database compiled in.
## Install
```bash
pip install renkin
```
No RDKit, no Boost, no C/C++ compiler needed at install time — RENKIN's
chemistry layer ([`chematic`](https://docs.rs/chematic/)) and search engine
are both pure Rust, compiled ahead of time into the wheel.
## A Working Example
```python
--8<-- "examples/quickstart.py"
```
`find_routes` always returns a **JSON string**, not a `dict` — call
`json.loads()` on it. Full parameter list and return shape:
[Python API reference](../api/python.md).
## Custom Building Blocks
With no `building_blocks` argument, RENKIN searches against `data/building_blocks.smi`
(402 unique compounds) *if that path resolves relative to your current working
directory* — in practice, only when running from a checkout of this repo. A
`pip install renkin` wheel does not bundle that file, so a plain `pip install`
run from anywhere else silently falls back to a smaller, compiled-in
152-compound set instead. Don't rely on either default having a specific
compound — supply your own stock explicitly:
```python
import renkin, json
my_stock = ["CC(=O)O", "Oc1ccccc1", "c1ccccc1", "Brc1ccccc1", "OB(O)c1ccccc1"]
result = json.loads(renkin.find_routes(
target="c1ccc(-c2ccccc2)cc1",
building_blocks=my_stock,
depth=3,
))
```
Any SMILES that fails to parse is silently skipped, not an error — it just
can't match as a leaf building block.
## Extracted Templates
The built-in rule set is 28 hand-crafted, human-readable disconnections
(ester cleavage, Suzuki, Buchwald-Hartwig, and so on). For broader reaction
coverage, load additional SMIRKS templates auto-extracted from USPTO-50k/MIT
via rdchiral:
```python
result = json.loads(renkin.find_routes(
target="CC(=O)Oc1ccccc1C(=O)O",
templates_path="data/templates_extracted_5000.smi",
depth=5,
))
```
Each extracted template gets a stable `template_id`
(`smirks-sha256:<hex>`) derived from the SMIRKS itself, independent of file
order or position — unlike the display name (`extracted_0`, `extracted_1`, ...),
which shifts if the file is re-sorted or re-extracted.
## Evidence Metadata (Conditions, Yields, References)
You can attach curated external evidence — reported conditions, yields, DOIs,
patents, known side-reaction warnings — to a specific template, keyed by its
`template_id`:
```python
result = json.loads(renkin.find_routes(
target="CC(=O)Oc1ccccc1C(=O)O",
templates_path="data/templates_extracted_5000.smi",
template_metadata_path="sidecar.json",
depth=5,
))
for route in result["routes"]:
for step in route["steps"]:
if "evidence" in step:
print(step["template_id"], step["evidence"])
```
Steps whose template has no matching sidecar entry simply have no `evidence`
key — nothing is fabricated. See the [Reaction Evidence Metadata
guide](reaction-evidence.md) for the sidecar format and what `evidence` is
(and isn't).
## Reading the Result
Real output for aspirin at `depth=1` (hand-crafted rules only, one route shown):
```python
{
"depth": 1,
"score": 1.099087,
"confidence": 1.0,
"success_probability": 1.0,
"route_cost": 8.298266666666667,
"building_blocks": ["OC(=O)C", "c1cccc(c1O)C(O)=O"],
"steps": [
{
"target": "OC(=O)c1ccccc1OC(=O)C",
"rule": "ester_cleavage",
"template_id": "rule:ester_cleavage",
"precursors": ["OC(=O)C", "c1cccc(c1O)C(O)=O"],
"step_confidence": 1.0,
"atom_economy": 90.90950376941474,
"atom_economy_raw_percent": 90.90950376941474,
"atom_economy_status": "normal",
"reaction_family": "esterification",
"conditions": {"catalyst": "NaOH or LiOH (2 eq)", "solvent": "THF/H₂O (2:1)", "temperature": "rt → 60 °C"},
"procedure_hint": "Dissolve in THF/H₂O, add NaOH (2 eq), stir at 60 °C, acidify to pH 2.",
"metadata_source": "handcrafted_default",
"metadata_scope": "reaction_family"
# evidence appears only when a --template-metadata sidecar matches this template_id
}
]
}
```
`step_confidence`/`success_probability` are template-frequency-derived
search-ranking scores (here 1.0 because, with only hand-crafted rules loaded,
every rule has equal weight) — not a measured or predicted experimental
yield. `conditions`/`procedure_hint` are rule-author-supplied defaults for
hand-crafted rules (`metadata_source: "handcrafted_default"`), not a literature
citation — see [Reaction Evidence Metadata](reaction-evidence.md) for the
distinction and how to attach real cited evidence.
## Current Limitations
- The default stock (402 compounds when running from a repo checkout, 152
otherwise — see [Building Blocks](../api/python.md#building-blocks)) and 28
hand-crafted rules cover common pharmaceutical disconnections well, but broader reaction space needs
the larger extracted-template files or your own stock.
- No literature/patent auto-search, no automatic side-reaction prediction, no
yield prediction — see [Reaction Evidence Metadata](reaction-evidence.md)
for exactly what curated evidence is and isn't.
- Current benchmark numbers are still being re-measured after a validator fix
— see the [Benchmark page](../benchmark.md) before citing a success rate.
## Next Steps
- [Python API reference](../api/python.md) — full parameter list, `predict_forward`, `validate_forward`
- [Reaction Evidence Metadata](reaction-evidence.md) — conditions, yields, references, warnings
- [Rust API](../api/rust.md) / [WASM API](../api/wasm.md) — if you need the engine outside Python